The Halides of Niobium and Tantalum F. F A I R B R O T H ER The University of Manchester, England

The Halides of Niobium and Tantalum

F . F A I R B R O T H E R

The University of Manchester, England 1. Fluorine Compounds

A . Pentafluorides B . Sub-fluorides 2. Chlorine Compounds

A . Pentachlorides B . Sub-chlorides 3. Bromine Compounds

A. P e n t a b r o m i d e s B . Sub-bromides 4. Iodine Compounds

A. Pentaiodides B . Sub-iodides References

123 123 132 134 134 151 160 160 164 166 167 170 174

1. Fluorine Compounds A. Pentafluorides

(i) Preparation

(a) Direct fluorination of the metals. Gaseous fluorine is passed over t h e p o w d e r e d m e t a l w h i c h is h e a t e d t o a b o u t 250° i n a Monel-metal or nickel r e a c t o r (Priest, 1950; F a i r b r o t h e r a n d F r i t h , 1951).

(b) Action of gaseous hydrogen fluoride on the metals at 300° (Emeleus a n d G u t m a n n , 1950). W i t h n i o b i u m , t h i s r e a c t i o n a t a t m o s p h e r i c pres­

sure yields only t h e pentafluoride, b u t w i t h t a n t a l u m some involatile trifluoride is also p r o d u c e d ; a t 225° a n d high pressures in a sealed vessel, b o t h m e t a l s give a m i x t u r e of pentafluoride a n d trifluoride (Muetterties a n d Castle, 1961).

(c) Addition of bromine trifluoride. This is cautiously a d d e d t o a cooled suspension of t h e m e t a l p o w d e r or p e n t o x i d e in b r o m i n e , followed b y t h e t h e r m a l decomposition of t h e N h B r F g ^ F ^ or TaBrFg"^ F ^ formed in t h i s reaction ( G u t m a n n a n d E m e l e u s , 1950).

1 2 3

124 Γ. FAIRBROTHER

(d) Action of tin{II) fluoride on the metal

5SnF2 + 2 N b - > 5Sn + 2 N b F 5

T h i s m e t h o d has been used so f a r only for t h e preparation o f N b F g (Gortsema a n d D i d c h e n k o , 1965). T h e powdered reactants are heated i n a m o l y b d e n u m crucible inside a n Inconel reactor under a cover of argon. T h e pentafluoride is produced a t temperatures between 375 a n d 500°, sublimes o u t of t h e reaction m i x t u r e a n d is condensed on t h e cooled l i d of t h e I n c o n e l reactor.

(e) Action of liquid hydrogen fluoride on the pentachlorides ( R u f f a n d Schiller, 1 9 1 1 ; B r a u e r , 1963). T h e pentachloride is dissolved i n t h e hydrogen fluoride a n d t h e solution w a r m e d under a well-cooled m e t a l reflux condenser u n t i l t h e evolution of hydrogen chloride ceases; t h e excess o f hydrogen fluoride is distilled off a n d t h e product purified b y sublimation.

(f) Action of zinc fluoride on anhydrous niobium pentachloride at 300°

(Niehues i n Schafer et al., 1965b)

mCl, + 2-5 Z n F g = N b F ^ + 2-5 ZnCls

(g) Thermal decomposition at 900° of the double barium-tantalum fluoride ( H a h n a n d P u t t e r , 1923).

(h) Reaction between aluminium fluoride and K^NbF^j at 800° (Schafer et al., 1965b). T h e K g N b F y is heated w i t h a l u m i n i u m fluoride i n a nickel b o a t i n a stream o f d r y nitrogen.

K g N b F ^ +

2AIF3

- > N b F s +

2KAIF4

T h e reaction is a slow one (8 h ) , t h e pentafluoride subliming i n t h e nitrogen stream a n d t h e

KAIF4

remaining i n t h e reaction boat. K g T a F y w o u l d p r o b a b l y react i n a n analogous manner.

(j) Puriflcation. T h e pentafluorides are e x t r e m e l y hygroscopic a n d deliquesce on exposure t o t h e a i r ; t h e y m a y be purifled b y sublimation in vacuo i n glass apparatus, provided t h a t this has been well out-gassed.

P y r e x glass is scarcely a t t a c k e d b y t h e pentafluorides a t their m e l t i n g points or a t 120° a t w h i c h t h e v a p o u r pressures are a b o u t 20 m m (Fair- brother a n d F r i t h , 1951). W h e n Hquid N b F g is heated a t 1 4 0 - 1 6 0 ° i n J e n a Gerateglas, t h e l a t t e r becomes cloudy; b y contrast, transparent quartz is n o t noticeably a t t a c k e d a t 300°, even after 10 days. A t > 4 0 0 ° , however, a reaction takes place w i t h t h e f o r m a t i o n o f N b O g F (Schafer et al, 1965b).

SiOa + N b F ^ - N b O ^ F + SiF^

THE HALIDES OF NIOBIUM AND TANTALUM 125

a(A) 6(A) c(A) _β

NbF^ 9-62 ± 0-01 14-43 ± 0 0 2 5-12 db 0-01 96-1° ± 0-3°

TaFs 9-64 ± 0-01 14-45 ± 0 0 2 5-12 d= 0-01 96-3° ± 0-3°

u n i t is a t e t r a m e r in which t h e m e t a l a t o m s are s i t u a t e d a t t h e corners of a s q u a r e a n d are linked b y bridging fluorine a t o m s . T h e r e are t w o metal-fluorine distances: (a) m e t a l t o bridging-fluorine a t o m 2-06Â;

(b) m e t a l t o non-bridging fluorine a t o m 1-77 Â ( E d w a r d s , 1964). T h e density values (g cm-^) a r e : NbPg, 3-54 (X-ray) a n d 3-29 (measured);

TaFg, 5-19 (X-ray) a n d 4-98 (measured).

T h e melting points a r e : NbFg, 80-0° ( F a i r b r o t h e r a n d F r i t h , 1951), o t h e r values 75-5° (Ruff a n d Schiller, 1911), 78-9° ( J u n k i n s et al., 1952), 79° (Schafer et al., 1965b); T a F 5 , 95-1° ( F a i r b r o t h e r a n d F r i t h , 1951), o t h e r values 96-8° (Ruff a n d Schiller, 1911), 97° (Schafer et al., 1965b).

T h e liquid pentafluorides are m o d e r a t e l y viscous, w i t h densities a n d viscosities (Table II) a t t h e melting p o i n t s (i°) ( F a i r b r o t h e r et al., 1965a).

T A B L E I I . D e n s i t i e s a n d v i s c o s i t i e s o f p e n t a f l u o r i d e s

NbFg T a F s

D e n s i t y d\ 2-6995 3-8800

(g c m- 3 )

V i s c o s i t y η 91-41 70-31

( c e n t i p o i s e s )

These viscosities, which are a b o u t 2 5 0 t i m e s t h e viscosity of w a t e r a t t h e s a m e t e m p e r a t u r e s , indicate a high degree of molecular c o m p l e x i t y in t h e melt. This is s u p p o r t e d b y t h e high T r o u t o n c o n s t a n t s , w h i c h a r e 2 5 - 4 cal (deg K ) - i for N b F g a n d 2 5 - 9 cal (deg K ) - i for TaFg respectively ( F a i r b r o t h e r a n d F r i t h , 1 9 5 1 ) .

T h e m o l t e n pentafluorides possess m e a s u r a b l e electrical conductivi­

ties ( F a i r b r o t h e r et al., 1 9 5 4 ) . These conductivities, in conjunction w i t h t h e densities a n d viscosities, indicate, however, a self-ionization of t h e melts of less t h a n 1 % .

(ii) Physical Properties

T h e pentafluorides are dense w h i t e sohds t h a t crystallize in t h e m o n o - clinic s y s t e m , in t h e space g r o u p (72/m, w i t h t h e u n i t cell dimensions given in T a b l e I a n d eight formula u n i t s in t h e u n i t cell. T h e s t r u c t u r a l

T A B L E I . U n i t cell d i m e n s i o n s o f p e n t a f l u o r i d e s

1 2 6 F. FAIRBROTHER

T h e vapour pressures of t h e Uquid pentafluorides are, for N b F g l o g^ m m = 8-439 - 2 8 2 4 / T

a n d for T a F .

log i ) m m = 8- 5 2 4 - 2 8 3 4 / Γ

(Fairbrother a n d F r i t h , 1 9 5 1 , Junkins et al., 1952). T h e boiling points (760 m m ) are N b F g 234-9° a n d T a F s 229-2°.

There is little difference between the vapour pressures of the p e n t a ­ fluorides over t h e whole range of l i q u i d i t y , b u t a t a n y given t e m p e r a t u r e the vapour pressure of t h e lighter N b F g is less t h a n t h a t of t h e heavier T a F ^ .

(iii) Chemical Properties

N i o b i u m ( V ) a n d t a n t a l u m ( V ) fluorides are strong Lewis acids, cata­

lyse Friedel-Crafts t y p e reactions a n d f o r m adducts w i t h a v a r i e t y o f n e u t r a l a n d anionic ligands. T h e y f o r m stable 1:1 adducts w i t h ethers a n d d i a l k y l sulphides, e.g. N b F s . M c s O ; NbFg.McgS; TaFg.McgO;

TaF5.Me2S; N b F s . E t ^ O ; NbFs.Et^S; T a F g . E t s O ; TaF5.Et2S. T h e flrst four of these are solid a t r o o m t e m p e r a t u r e a n d m e l t below 100°; the remaining four are liquids. T h e y can all be distilled unchanged a t low pressures, a n y slight decomposition w h i c h occurs being into penta­

fluoride a n d ligand, w i t h no replacement of fluorine b y oxygen t a k i n g place on heating. T e t r a h y d r o t h i o p h e n forms stable 1:2 adducts, while d i m e t h y l ether a n d d i m e t h y l sulphide f o r m 1:2 adducts t h a t are stable only a t low temperatures (Fairbrother et al., 1965b).

B r o m i n e trifluoride forms t h e adducts N b F g . B r F g a n d TaFg.BrFg w h i c h behave as salts MF6~.BrF2+, i n t h e bromine trifluoride solvent system ( G u t m a n n a n d Emeleus, 1950).

N i o b i u m ( V ) fluoride also reacts w i t h pyridine (Clark a n d Emeleus, 1958), w i t h a m m o n i a , a n d w i t h ethylenediamine (en) (Cavell a n d Clark, 1 9 6 1 ) to f o r m NbF5.2Py, N b F 5 . 2 N H 3 and N b F s . l - e e n respectively. O f the corresponding t a n t a l u m complexes, only the pyridine complex appears t o have been prepared. I t is n o t e w o r t h y t h a t , i n these reactions w i t h nitrogen bases, the n i o b i u m ( V ) fluoride undergoes neither a m m o - nolysis w i t h a m m o n i a nor reduction t o n i o b i u m ( I V ) w i t h pyridine, such as occur w i t h other pentahalides. Dimethylsulphoxide reacts w i t h the pentafluorides t o give the adducts NbF5.2Me2SO a n d TaF5.2Me2SO, which m e l t a t 44° a n d 63-5° respectively, w i t h o u t decomposition, giving electrically conducting liquids w i t h viscosities comparable w i t h those of the parent fluorides (Fairbrother et al., 1966). This behaviour is i n contrast to t h a t of the pentachlorides a n d pentabromides, w h i c h react w i t h dimethylsulphoxide a t room temperature to give polymeric o x y t r i - halide complexes (Copley et al., 1964a).

THE HALIDES OF NIOBIUM AND TANTALUM 127 N i o b i u m ( V ) a n d t a n t a l u m ( V ) fluorides also react w i t h sulphur t r i -

oxide, t o f o r m t h e addition compounds N b F 5. 2- l S 0 3 a n d TaF5.2-6S03.

These are probably t h e fluorosulphates NbF3(S03F)2 a n d TaF3(S03F)2 f r o m w h i c h t h e excess of SO3 has n o t been completely removed. T h e y decompose a t 1 7 5 - 2 2 5 ° to give pyrosulphuryl fluoride, t h e presumed re­

actions being:

MF3 + 2SO3 ->

MF3(S03F)2 ->

MOF3 + S2O5F2

(Clark a n d Emeleus, 1958).

T a n t a l u m ( V ) fluoride reacts w i t h xenon tetrafluoride t o give t h e straw-coloured complex XeF2.2TaF5 ( E d w a r d s et al., 1963).

N i o b i u m ( V ) a n d t a n t a l u m ( V ) fluorides f o r m double salts w i t h most ionic fluorides; although such double salts can be obtained b y direct addition i n t h e m o l t e n state, t h e y are more usually obtained b y t h e addition of a n ionic fluoride t o a solution of t h e hydrous oxide i n aqueous hydrofluoric acid. I n appropriate circumstances crystalline salts containing MFg", M F ^ ^ - , MFg^-,

MF5O2-

or MFgO^- m a y be obtained. A n u m b e r o f these salts were prepared b y Marignac (1866a) a n d b y B a l k e a n d S m i t h (1908).

T h e commercially most i m p o r t a n t salts of these series are potassium pentafluoroxoniobate(V), KaNbOFgjHaO, a n d potassium heptafluoro- t a n t a l a t e ( V ) , K2TaF7. These t w o salts f o r m e d t h e basis of t h e flrst successful separation of t a n t a l u m f r o m n i o b i u m (Marignac, 1866b), b y means of w h i c h , u n t i l f a i r l y recently, most pure t a n t a l u m was still extracted. T h e post W o r l d W a r I I d e m a n d for pure n i o b i u m , however, has led t o t h e development o f l i q u i d - l i q u i d extraction methods, for b o t h n i o b i u m a n d t a n t a l u m , w h i c h depend u p o n t h e p a r t i t i o n o f fluorocom- plexes of the metals between aqueous hydrofluoric-hydrochloric acid m i x ­ tures a n d methylisobutyl ketone ( W e r n i n g et al., 1954 ; Soisson ei aZ., 1961 ).

A n u m b e r of different crystalline products m a y be obtained f r o m t h e systems ( N b / T a) 2 0 5- I I F - R F- H 2 0 ( R = alkali-metal or a m m o n i u m ) , a n d detailed studies have been made o f t h e solubility relationships i n these systems (Savchenko a n d T a n a n a e v , 1946, 1947). T h e salts K 2 N b O F 5. H 2 0 a n d K2TaF7 separate f r o m a solution w h i c h is 1-2 molar i n H F . K 2 N b O F 5. H 2 0 can be recrystallized unchanged f r o m w a t e r a n d is stable i n hydrofluoric acid u p t o a concentration of about 5 % (2-5 M ) , above w h i c h K2NbF7 separates. I t is possible t o recrystallize K2TaF7 f r o m water, provided t h a t t h e operation is carried o u t quickly, b u t t h e K2TaF7 is slowly hydrolysed i n w a t e r t o give a n insoluble basic fluoro- t a n t a l a t e o f indeterminate composition; t h e hydrolysis is reversed b y the addition o f hydrofluoric acid. T h e solubility o f KaNbOFs.HgO i n 1 % hydrofluoric acid a t 20°C is about 12 times t h a t of K2TaF7, t h e

128 F. FAIRBROTHER

C a t i o n N a + K + R b + Cs+

Ba2+

S t r u c t u r e t y p e N a C l C s C l t r i g . t r i g .

α ( A ) 8-27 1 0 - 2 9 5-14 5-32

β — — ^{9 6 - 4 ° 9 5 - 8 °}

fractional crystallization of these salts being further facilitated b y t h e fact t h a t t h e solubiUty of t h e t a n t a l u m salt is considerably reduced b y t h e presence of excess K F in t h e solution or b y t h e presence of t h e n i o b i u m salt, whereas t h e solubility of t h e n i o b i u m salt is less affected b y t h e presence either of K F or t h e t a n t a l u m salt,

KgNbOFg.HgO crystallizes in t h i n monoclinic scales which are greasy t o t h e t o u c h : a\hx = 0-992:1:0-980; ή = 103°46'. T h e n i o b i u m a t o m is octahedrally coordinated b y t h e oxygen a n d t h e five fluorine a t o m s ( H o a r d a n d M a r t i n , 1941). K2TaF7 is also monoclinic, b u t crystalhzes in t h e form of needles: a:b:c = 0-4625:1:0·6709; ή = 90°. T h e s t r u c t u r e is so close t o being o r t h o r h o m b i c , however, t h a t it was t h o u g h t t o be so b y Marignac. T h e t a n t a l u m a t o m in t h e crystal is s i t u a t e d a t t h e centre of a slightly d i s t o r t e d trigonal prism of six fluorine a t o m s , t h e s e v e n t h fluorine a t o m being s i t u a t e d b e y o n d t h e centre of one of t h e r e c t a n g u l a r faces of t h e p r i s m ; t h e s a m e s t r u c t u r e is found in t h e i s o m o r p h o u s KgNbF^ (Hoard, 1939).

Hexafluoroniobates a n d h e x a f l u o r o t a n t a l a t e s s e p a r a t e from solution a t v e r y high concentrations of hydrofluoric acid. T h u s , K N b F g is in equilibrium w i t h t h e solution in t h e s y s t e m : K 2 N b F 7 - H F - H 2 0 w h e n t h e H F c o n c e n t r a t i o n Hes b e t w e e n 40-9 a n d 5 9 % ('-22-35 M), whilst K T a F g separates in a corresponding m a n n e r w h e n t h e hydrofluoric acid concentration exceeds 4 5 - 3 % (Savchenko a n d T a n a n a e v , 1946, 1947).

T h e hexafluoro salts m a y be m o r e conveniently obtained, however, b y t h e a n h y d r o u s reactions of p o t a s s i u m fluoride w i t h solutions of t h e m e t a l s or their p e n t o x i d e s in b r o m i n e trifluoride ( G u t m a n n a n d E m e l e u s , 1950), t h e reactions being essentially acid-base neutraliza­

tions in t h e b r o m i n e trifluoride s y s t e m :

BrF2+ N b F e " + K + B r F ^ - = K N b F e + 2BrF3

X - R a y p o w d e r p h o t o g r a p h s of a series of alkaH-metal a n d b a r i u m hexafluoroniobates a n d hexafluorotantalates p r e p a r e d b y t h i s m e t h o d showed (Table III) t h a t t h e t w o series are isomorphous a n d isodimen- sional t o w i t h i n v e r y n a r r o w limits ( ± 0 - 0 1 A) (Cox, 1956).

T h e X - r a y p o w d e r p h o t o g r a p h s of t h e l i t h i u m salts showed t h a t these also are i s o m o r p h o u s a n d isodimensional.

T A B L E I I I . H e x a f l u o r o n i o b a t e s a n d h e x a f l u o r o t a n t a l a t e s ( a n i o n N b F e " or TaFg")

THE HALIDES OF NIOBIUM AND TANTALUM 129

Silver hexafluoroniobate a n d silver h e x a f l u o r o t a n t a l a t e a r e soluble in benzene, t o l u e n e a n d m-xylene, t h e solubilities being t h e g r e a t e s t in t o l u e n e a n d least in benzene. O n e v a p o r a t i n g benzene solutions of t h e s e salts in a s t r e a m of d r y n i t r o g e n a t r o o m t e m p e r a t u r e , t h e complexes AgNbP6.2C6H6 a n d AgTaF6.2C6H6 are o b t a i n e d . Silver hexafluoro­

n i o b a t e a n d h e x a f l u o r o t a n t a l a t e ( b o t h of w h i c h possess t h e caesium chloride s t r u c t u r e , w i t h a = 9-93 Â), a r e i s o m o r p h o u s w i t h t h e corre­

s p o n d i n g p o t a s s i u m salts (Sharp a n d S h a r p e , 1956a). W h e n t h e silver salts are s h a k e n w i t h copper p o w d e r in t o l u e n e , t h e silver is q u a n t i t a ­ t i v e l y r e p l a c e d w i t h t h e f o r m a t i o n of solutions of copper(I) hexafluoro­

n i o b a t e or h e x a f l u o r o t a n t a l a t e (Sharp a n d S h a r p e , 1956b).

Octafluoroniobates a n d o c t a f l u o r o t a n t a l a t e s are less well k n o w n t h a n t h e h e p t a a n d h e x a complexes. Only sodium octafluoroniobate NagNhPg ( H o a r d a n d M a r t i n , 1941) a n d a m m o n i u m , s o d i u m (Balke,

1905; H o a r d et al., 1954; S a v c h e n k o a n d T a n a n a e v , 1946) a n d b a r i u m ( H a h n a n d P u t t e r , 1923) o c t a f l u o r o t a n t a l a t e s a p p e a r t o h a v e b e e n characterized. These salts s e p a r a t e from solutions w h i c h c o n t a i n t h e correct a m o u n t of n e u t r a l fluoride a n d w i t h i n a n a r r o w r a n g e of h y d r o ­ fluoric acid c o n c e n t r a t i o n w h i c h is j u s t sufiicient t o p r e v e n t hydrolysis.

T h e configuration of t h e TaFg^" ion is t h a t of a s q u a r e a n t i - p r i s m of fluorine a t o m s s u r r o u n d i n g a central m e t a l a t o m ( H o a r d et al., 1954).

T h e composition of t h e p a r t i c u l a r fluoroniobate or fluorotantalate w h i c h s e p a r a t e s from solution, however, is n o t a l w a y s i n d i c a t i v e of t h e relative p r o p o r t i o n s of t h e ionic species a c t u a l l y p r e s e n t in t h e solution.

Keller (1963) h a s c o m p a r e d t h e R a m a n s p e c t r a of KgNbOFg.HaO dis­

solved in w a t e r a n d K2NbF7 dissolved in 1-50% hydrofluoric acid, w i t h t h e R a m a n s p e c t r a of crystalline KgNbOFg.HgO, CsNbFg, a n d K2NbF7, a n d h a s s h o w n t h a t t h e N b O F g ^ - a n d N b F g - ions a r e p r e s e n t in t h e s e solutions, b u t t h a t t h e ion NbF72^ could n o t b e d e t e c t e d in solution. B y a similar m e t h o d , Keller a n d C h e t h a m - S t r o d e (1966) h a v e s h o w n t h a t in solutions ^^1 M in T a , r^ z e r o 1 M in H F a n d 5-42 M in

NH4F,

t h e p r e d o m i n a n t species w a s t h e TaF^^- ion, whilst in solutions which were ^^1 M in T a a n d ^ 2 4 M in H F , t h e TaFg- ion w a s t h e p r e d o m i n a n t species, w i t h TaF72- b a r e l y d e t e c t a b l e . A t i n t e r m e d i a t e c o n c e n t r a t i o n s of H F , b o t h T a F ^ - a n d TaF72- were p r e s e n t b u t TaFg^- w a s n o t d e t e c t e d in t h e s e solutions. Also, P a c k e r a n d M u e t t e r t i e s (1963), e x a m i n i n g t h e nuclear m a g n e t i c resonance of ^^Nb a n d ^^F, h a v e shown t h a t N b F g - is p r e s e n t in acetonitrile solutions of t h e r e a c t i o n p r o d u c t of niobium(V) fluoride w i t h d i m e t h y l f o r m a m i d e , b u t t h a t it shows little t e n d e n c y t o i n t e r a c t w i t h F - t o give NbF72-.

N u c l e a r m a g n e t i c resonance studies h a v e also b e e n m a d e of solutions of niobium(V) fluoride in d r y e t h a n o l ( H a t t o n et al., 1965).

130 F. FAIRBROTHER

R a m a n a n d nuclear m a g n e t i c resonance spectra, however, i n d i c a t e chiefly t h e p r e d o m i n a n t species. P o t e n t i o m e t r i c studies (Varga a n d F r e u n d , 1962; B u k h s h a n d M a d d o c k , 1965) carried o u t over a wide r a n g e of acid a n d fluoride concentrations, indicate t h a t in all a q u e o u s solutions which contain niobium(V) or t a n t a l u m ( V ) fluoride a n d fluoride ion, t h e r e exists a d i s t r i b u t i o n of t h e species MF^^-'^ where ^ < 8, t o g e t h e r ivith related oxo or h y d r o x o anions, especially a t low acid concentrations. These results are in a g r e e m e n t w i t h t h e empirical obser­

v a t i o n t h a t octofluoroniobates a n d octofluorotantalates are o b t a i n e d from solutions which contain only a small c o n c e n t r a t i o n of H F , whilst hexafluoroniobates a n d h e x a f l u o r o t a n t a l a t e s are o b t a i n e d from highly acid solutions. B u k h s h a n d M a d d o c k h a v e re-measured t h e formation c o n s t a n t s of t h e fluorotantalate ions, a n d find: = 4 - 6 χ 10^; kη

= 1-26 X 10^ a n d k^ = 4-5 a t 25°. N o evidence of t h e existence of T a F g ^ - w a s found.

Hexafiuoroxoniobates a n d hexafluoroxotantalates exist in addition t o t h e pentafluoroxoniobates, e.g. KgNbOFg.HaO, a n d a few, b u t less easily formed p e n t a f l u o r o x o t a n t a l a t e s (the p r e p a r a t i o n of well defined fiuoroxotantalates is m o r e difficult on a c c o u n t of t h e n a r r o w range of hydrofiuoric acid concentration within which t h e y are stable). F o r example, KgNbOFg a n d KgNbOFg-HgO are o b t a i n e d on t h e a d d i t i o n of excess p o t a s s i u m fluoride t o solutions of KgNbOFg.HgO ( H o a r d a n d Martin, 1941). I n these c o m p o u n d s , which are less stable t h a n t h e p e n t a - fluoroxo salts, t h e conflguration of t h e 7-coordinated anion is different from t h a t of N b F ^ ^ - or TaF7^~, t h e additional fluorine a t o m being sited b e y o n d t h e centre of one of t h e octagonal faces of t h e N b O F g ^ - ion;

in this respect t h e s t r u c t u r e is similar t o t h a t of ZrF7'^- in K3ZrF7 (Williams a n d H o a r d , 1942).

T h e crystal s t r u c t u r e of KgNbOFg is consistent w i t h t h e presence in t h e solid of n i o b i u m - o x y g e n double b o n d s a n d a b a n d a t 922 cm~i is found in its infrared s p e c t r u m ; a similar b a n d is found in t h e infrared s p e c t r u m of K2NbOF5,H20 (Field a n d H a r d y , 1963). These b a n d s a r e in t h e region considered t o be diagnostic for m e t a l - o x y g e n double b o n d s (Barraclough et al., 1959). N o b a n d in this region is found in t h e infrared s p e c t r u m of K2NbF7.

T h e acid heptafluoroxoniobate K 3 H N b O F 7 , w h i c h is o b t a i n e d b y dis­

solving n i o b i u m p e n t o x i d e in t h e correct excess of hydrofluoric acid, t h e n a d d i n g p o t a s s i u m c a r b o n a t e until t h e solution is only slightly acidic a n d recrystallizing t h e p r o d u c t from dilute hydrofluoric acid (Balke a n d S m i t h , 1908), is a double salt, K 2 N b O F 5 . K H F 2 ; it consists of a lattice aggregate of K + ions, linear H F g " ions, a n d o c t a h e d r a l l y co-ordinated NbOFg^- ions ( H o a r d a n d Martin, 1941).

THE HALIDES OF NIOBIUM AND TANTALUM 1 3 1

M{V) Oxidefluorides. Niobium(V) a n d t a n t a l u m ( V ) oxidetrifluorides h a v e n o t b e e n fully characterized as definite c o m p o u n d s , t h o u g h a m o r ­ p h o u s p r o d u c t s w i t h compositions a p p r o x i m a t i n g t o M O F 3 h a v e b e e n described from t i m e t o t i m e , a n d t h e y m a y be p r e s u m e d t o b e f o r m e d as i n t e r m e d i a t e hydrolysis p r o d u c t s of t h e pentafiuorides, or pyrolysis p r o d u c t s of t h e i r s u l p h u r trioxide a d d u c t s (Clark a n d E m e l e u s , 1 9 5 8 ) ; T a O P g h a s also b e e n r e p o r t e d as a p r o d u c t of t h e t h e r m a l r e a c t i o n b e ­ t w e e n t a n t a l u m ( V ) fluoride a n d silica (Schafer et al., 1 9 6 4 a ) .

On t h e o t h e r h a n d , t h e p r o d u c t s of further hydrolysis, N b O g F a n d

T a O g F are o b t a i n e d as crystalline species w h e n solutions of t h e m e t a l s or t h e p e n t o x i d e s in 4 8 % a q u e o u s hydrofluoric acid are e v a p o r a t e d t o d r y n e s s a n d t h e residue h e a t e d t o 2 5 0 ° . X - R a y p o w d e r p h o t o g r a p h s indicate t h a t these dioxyfluorides possess t h e R e O g s t r u c t u r e in w h i c h t h e fluorine a t o m s a n d oxygen a t o m s are r a n d o m l y d i s t r i b u t e d in o c t a h e d r a l positions a b o u t t h e m e t a l a t o m s . T h e simple cubic u n i t cell for N b O g F h a s : α = 3 - 9 0 2 ± 0 - 0 0 1 Â a n d for ΎΜ^Έ, α - 3 - 8 9 6

± 0 - 0 0 3 Â (Frevel a n d R i n n , 1 9 5 6 ) .

N b O g F c a n b e t r a n s p o r t e d b y diffusion-convection w h e n h e a t e d in a sealed t u b e in t h e presence of NbClg, in a t e m p e r a t u r e g r a d i e n t of 4 0 0 3 0 0 deg. Since t h e N b O g F is deposited a t t h e lower t e m p e r a t u r e , t h e t r a n s p o r t r e a c t i o n is e n d o t h e r m i c a n d suggests t h a t one of t h e gaseous c o m p o u n d s NbOFClg or NbOFgCl (which are u n k n o w n a t p r e s e n t ) , or N b O F g , is p r e s e n t :

N b 0 2 F ( s ) + mChig) = NbOFCl2(g) - f NbOCl3(g)

If t h i s t r a n s p o r t is carried o u t in t h e presence also of a t r a c e of chlorine, t h e N b O g F crystals which are o b t a i n e d are colourless; in t h e absence of chlorine t h e y are coloured blue, which indicates t h a t some r e d u c t i o n t o N b ( I V ) h a s t a k e n place (Schafer, 1964).

O t h e r oxidefluorides of niobium(V), N b 3 0 7 F a n d NbgOigF m a y be o b t a i n e d b y h e a t i n g m i x t u r e s of N b O g F a n d NbgOg (Andersson a n d  s t r o m , 1 9 6 4 ) . N b 3 0 7 F , which was first observed as a t r a c e in ''Spec- p u r e " n i o b i u m p e n t o x i d e , m a y be o b t a i n e d as single crystals, which are o r t h o r h o m b i c , w i t h space g r o u p (7^,^^^ a n d u n i t cell: a = 2 0 - 6 7  ; b = 3 - 8 3 3 A; c = 3 - 9 2 7 Â. T h e s t r u c t u r e consists of blocks w i t h t h e

R e 0 3 s t r u c t u r e , a t t a c h e d b y o c t a h e d r a sharing edges a n d e x t e n d i n g indefinitely in t h e b a n d c directions, b u t limited t o t h r e e in t h e a direction (Andersson, 1 9 6 4 ) .

T h e X - r a y p o w d e r d i a g r a m of N b g O^ g F shows t h a t its s t r u c t u r e is v e r y similar t o t h a t of y-NbgOs.

132 F. FAIRBROTHER

B. Sub-fluorides (i) Tetrafluorides

N i o b i u m ( I V ) fluoride, N b F 4 , has been prepared b y t h e reduction o f n i o b i u m ( V ) fluoride b y (a) sihcon powder, (b) n i o b i u m m e t a l powder (Gortsema a n d D i d c h e n k o , 1965), (c) n i o b i u m foil (Schafer et al., 1965b), under pressure i n a t h i c k - w a l l e d sealed quartz t u b e or m e t a l reaction vessel, a t temperatures between about 250 a n d 350°. N i o - b i u m ( I V ) fluoride is a black, involatile a n d v e r y hygroscopic solid;

w h e n left i n t h e air i t hydrolyses a n d is converted completely t o N b O g F . I t reacts vigorously w i t h w a t e r t o give a b r o w n solution a n d a b r o w n precipitate o f indeterminate composition.

T h e crystal structure o f N b F 4 differs f r o m t h a t of other n i o b i u m tetrahalides. Measurements of X - r a y powder diffraction patterns m a d e independently b y t h e above groups of workers agree excellently a n d indicate a tetragonal u n i t cell w i t h : a = 4-082 ± 0-001 A; c = 8-161

± 0-0005 Â , a n d containing t w o N b F 4 units. T h e structure is unusual i n t h a t t h e cja ratio appears t o be exactly 2, w i t h i n t h e limits of experi­

m e n t a l accuracy. T h e structure is a combination o f a body-centred tetragonal n i o b i u m lattice a n d a face-centred cubic closest packed fluoride lattice ( F i g . 1).

T h e measured pycnometric density of 4-01 g cm"^ agrees well w i t h the calculated X - r a y density o f 4-13. T h e structure o f N b F 4 gives no

F I G . 1 . T h e unit cell of n i o b i u m tetrafluoride. (After Gortsema a n d D i d c h e n k o , 1 9 6 5 . )

THE HALIDES OF NIOBIUM AND TANTALUM 133 evidence of a n y m e t a l - m e t a l bonding such as occurs i n other n i o b i u m

a n d t a n t a l u m tetrahalides. This absence o f m e t a l - m e t a l bonds is sup­

ported b y t h e observation t h a t N b F 4 is paramagnetic. Schafer et al.

(1965b) have obtained values for t h e molar susceptibility o f X - + 1 7 5 e.m.u. a t 9 0 ° K a n d +200 e.m.u. a t 2 9 5 ° K (extrapolated t o

a n d uncorrected for diamagnetic contribution).

N i o b i u m ( I V ) fluoride is stable under v a c u u m u p t o a t e m p e r a t u r e of a p p r o x i m a t e l y 2 7 5 - 3 2 5 ° , b u t disproportionation takes place r a p i d l y above 350°. T h e products of this disproportionation are discussed below.

T a n t a l u m ( I V ) fluoride has n o t y e t been described.

(ii) Trifluorides and Lower Fluorides

A compound w i t h t h e composition N b F g was obtained b y E h r l i c h et al. (1956) f r o m t h e reaction of n i o b i u m hydride w i t h a h y d r o g e n - hydrogen fluoride m i x t u r e a t 5 6 0 - 5 8 0 ° . This compound was crystalline, w i t h a ReOg-type structure a n d a = 3-903 Â . Muetterties a n d Castle (1961), f r o m t h e reaction between n i o b i u m a n d hydrogen fluoride a t 225° under pressure, obtained a cubic product (a = 3-89 Â ) , w h i c h t h e y regarded as N b F g . O n t h e other h a n d , Schafer et al. (1965b) claim t h a t ' ' N b F g " can i n general only v e r y exceptionally be o b t a i n e d pure, a n d t h e n under still n o t clearly defined conditions, a n d as a metastable com­

pound. I t is stabilized b y traces o f oxygen i n t h e lattice a n d there are indications t h a t a stable series o f m i x e d crystals exists between t h e compositions N b O g F a n d NbO1.25F1.75 w i t h a continuous v a r i a t i o n of the cubic lattice constant. These compounds possess t h e blue colour t h a t has generally been observed i n preparations o f ' ' N b F g " . I t m a y bé added t h a t Gortsema a n d Didchenko (1965) observed t h a t t h e black t o blue-black solid product of t h e t h e r m a l decomposition of N b F 4 :

2NbF4 - N b F s + N b F g

could never be prepared completely free f r o m oxygen, t h e concentra­

t i o n of w h i c h , however, was usually less t h a n 1*5%.

T a n t a l u m trifluoride, TaFg, was obtained as a 1 0 % residue w h e n hydrogen fluoride was passed a t atmospheric pressure over t a n t a l u m a t 300°, t h e m a j o r product being t a n t a l u m ( V ) fluoride (Emeleus a n d G u t m a n n , 1950). I t crystallizes w i t h a ReOg-type cubic structure a = 3-90 Â ( G u t m a n n a n d J a c k , 1951). I t is also obtained, together w i t h T a F 5 , b y t h e action of hydrogen fluoride under pressure on t h e m e t a l a t 225° (Muetterties a n d Castle, 1961).

I f , i n t h e preparation of n i o b i u m ( I V ) fluoride b y heating n i o b i u m m e t a l w i t h a n excess of N b F g i n a closed t u b e , t h e t e m p e r a t u r e is m a i n t a i n e d a t , say 400°C, a t w h i c h t e m p e r a t u r e T a F 4 is unstable, t h e

134 Γ. FAIRBROTHER

end-product of t h e reaction is f o u n d t o have t h e composition NbFg. 5 (NbeFjs). This compound is more conveniently prepared b y enclosing n i o b i u m foil w i t h a n excess of n i o b i u m ( V ) fluoride under a n argon atmosphere, a t opposite ends of a thick-walled closed nickel t u b e ; t h e end of t h e tube t h a t contains t h e n i o b i u m is heated t o 900° a n d t h e other end t o 400°. A f t e r 4 days, t h e excess TaFg is r e m o v e d b y volatiliza­

t i o n a n d a crystalline residue of NbFg. 5 remains a t t h e cooler end of t h e t u b e (Schafer et al, 1965b).

I n contrast t o n i o b i u m ( I V ) fluoride, NbFg.g is stable w h e n exposed t o t h e air a n d is u n a t t a c k e d b y common m i n e r a l acids or alkalis, even w h e n heated. U n d e r v a c u u m , a t temperatures above 700° i t decom­

poses i n t o m e t a l a n d n i o b i u m ( V ) fluoride:

2NbF2.5 ==Nb + N b F 5

NbFg.s crystallizes i n t h e cubic system, w i t h a = 8-190 Â a n d a u n i t cell w h i c h contains 12 NbFg.s f o r m u l a units, or 2 NbgFjs units. T h e pycnometric density = 4-91 g c m - ^ agrees w i t h t h e X - r a y density of 5-09. T h e structure consists of polynuclear [Nb6Fi2]^"^ groups w h i c h are b o u n d together i n a continuous three-dimensional a r r a y b y t h e remaining fluoride ions. T h e core of the structure is a regular octahedron of n i o b i u m atoms i n w h i c h each n i o b i u m a t o m is situated equidistant f r o m four other n i o b i u m atoms a t 2-80 Â . A similar structure is f o u n d i n other sub-halides of n i o b i u m a n d t a n t a l u m o f general f o r m u l a

M6X14

a n d is associated w i t h a strong m e t a l - m e t a l a t o m interaction.

T h i s compound is t h e first fluoride t o be prepared w h i c h contains such m e t a l - m e t a l bonded clusters.

2. Chlorine Compounds A. Pentachlorides

(i) Preparation

N i o b i u m ( V ) a n d t a n t a l u m ( V ) chlorides have been prepared b y a v a r i e t y of methods, t h e chief of which are given below; a fuller list of references is given elsewhere (Fairbrother, 1967).

(a) Direct chlorination of the metal by gaseous chlorine d i l u t e d w i t h argon or nitrogen, a t 3 0 0 - 3 5 0 ° (Alexander a n d F a i r b r o t h e r , 1949). I n t h e case of t a n t a l u m ( V ) chloride, t h e chlorine m a y be replaced b y d r y hydrogen chloride ( Y o u n g a n d B r u b a k e r , 1952; Cowley et al, 1958) t h e tempera­

t u r e being raised t o about 410°; a t higher temperatures t h a n this some TaClg is produced, b u t is involatile. H y d r o g e n chloride is less satis­

factory for t h e preparation o f n i o b i u m ( V ) chloride, since even a t 300°

a m i x t u r e of pentachloride a n d trichloride is f o r m e d ( S p i t z y n a n d Preobrashenski, 1940).

THE HALIDES OF NIOBIUM AND TANTALUM 135 (b) Action of chlorine on a heated pentoxide-carbon mixture. This m e t h o d

has been w i d e l y used a n d is t h e original m e t h o d of preparation used b y Rose (1846). I n t h e case of n i o b i u m , however, t h e pentachloride is liable t o be c o n t a m i n a t e d b y oxidetrichloride; t h e product of the chlorination of a n i o b i u m p e n t o x i d e - c a r b o n m i x t u r e below 500° is chiefly NbOClg, a n d u p t o 1000° is always a m i x t u r e o f pentachloride a n d o x i d e t r i ­ chloride. T h e latter, however, can be converted into pentachloride b y passing t h e gaseous m i x t u r e , together w i t h excess chlorine, t h r o u g h a porous carbon plug heated a t 500° ( L i n d a n d Ingles, 1954). T h e pre­

p a r a t i o n of t a n t a l u m ( V ) chloride b y chlorination o f a pentoxide-carbon m i x t u r e , requires a somewhat higher t e m p e r a t u r e t h a n t h e preparation of n i o b i u m ( V ) chloride, b u t t h e product is less likely t o be contaminated b y oxidetrichloride.

(c) Action of other chlorinating agents on the pentoxides. These m a y be d i v i d e d i n t o (i) organic chlorinating agents (chiefly chlorocarbons), a n d (ii) inorganic chlorinating agents.

Carbon tetrachloride vapour, alone or m i x e d w i t h chlorine, or carbon tetrachloride i n t h e liquid f o r m i n a sealed t u b e , have been used b y m a n y workers for t h e chlorination o f either or b o t h pentoxides.

N i o b i u m pentoxide is more easily chlorinated t h a n is t a n t a l u m pent­

oxide. Chlorination of NbgOs begins a t 2 2 0 - 2 2 5 ° while t h a t o f TagOg does n o t begin below 270° ( R u f f a n d T h o m a s , 1926); however, i f t h e TagOg contains NbaOg, chlorination m a y t a k e place a t a substantially lower t e m p e r a t u r e t h a n 270°, especially i f t h e t w o oxides h a v e been precipitated together (Schafer et al., 1951b). Other chlorocarbons also m a y be used as chlorinating agents. F u l l y chlorinated b u t u n s a t u r a t e d chlorocarbons, a t a l l events u p t o hexachlorocyclopentadiene, appear to be less efficient chlorinating agents t h a n t h e saturated chlorocarbons ( A t k i n s o n et al., 1952). B a r d a w i l et al. (1965), however, have f o u n d t h a t n i o b i u m ( V ) chloride can be prepared i n good yield b y refluxing t h e pentoxide w i t h octochlorocyclopentene ( m . p . 3 8 ° ; b.p. 2 8 5 ° ) ; b y comparison, t a n t a l u m pentoxide showed l i t t l e or no reaction w i t h this compound under t h e same conditions.

Inorganic chlorination agents w h i c h have been used include phos­

phorus pentachloride—^which is n o t v e r y satisfactory—sulphur mono- chloride or dichloride a n d t h i o n y l chloride. T h e last n a m e d has been used b o t h t o chlorinate t h e anhydrous pentoxides i n a sealed t u b e a n d t h e hydrous oxides b y boiling under a reflux condenser; t h e most recent use o f t h i o n y l chloride, however, proves t o be one of t h e easiest routes t o t h e p r e p a r a t i o n of t h e pentachlorides f r o m t h e hydrous oxides (Bagnall a n d B r o w n , 1964). N i o b i u m h y d r o x i d e reacts vigorously w i t h t h i o n y l chloride a t room t e m p e r a t u r e , dissolving almost completely

136 F. FAIRBROTHER

within 24 h; the yield of pentachloride, after vacuum-evaporation of the solution and sublimation of the resulting solid, may be greater than 95%. In addition to the preparative simplicity of this method, the pentachloride is obtained free from NbOClg, which is almost always formed in other preparative procedures and is difficult to remove. The reaction is less efficient in the case of tantalum hydroxide, only about 6 0 % of the hydroxide dissolving in the thionyl chloride under the same conditions; this difiFerence is probably a result of the more rapid ageing of the tantalum hydroxide. Thionyl chloride does not react with the anhydrous pentoxides unless heated.

(ii) Physical Properties

Solid niobium(V) chloride is lemon-yellow in colour and pure tan- talum(V) chloride is white. Their densities, which are less than those of the corresponding pentafluorides, are: NbClg, df (pyc.) 2-75, X-ray 2-78: TaClg, df (pyc.) 3-68, X-ray 3-76. When sublimed under a vacuum at 200°, tantalum(V) chloride may condense as needles, but when solidi­

fied from a melt usually appears as a sharp-toothed mass; molten niobium(V) chloride also often solidifies in the same habit, but the very pure material crystallizes in large prisms which are almost orthorhombic and which show some evidence of undergoing a polymorphic change at about 183°, both by the appearance of white needles on the yellow prisms and by a change of slope of the log ρ against 1/T curve at this temperature (Alexander and Fairbrother, 1949). This polymorphic change is elusive, having been observed independently by several workers (private communications) and disclaimed by others; it is possible that it may be sluggish and retarded by traces of tan- talum(V) chloride. (The polymorphic change of y-NbgOg to a-NbgOg is retarded by the incorporation of small amounts of TagOg as a mixed phase.)

The crystal structure of niobium(V) chloride has been studied by single-crystal methods (Zalkin and Sands, 1958). The crystals are mono­

clinic, with a = 18-03 ± 0-01 A; b = 17-96 ± 0-02 A; c = 5-888

± 0-004 Â; ή = 90-60 dz 0-01°. The structure consists of NbaCliodimers, of which there are six in the unit cell; the chlorine atoms form two octahedra which share a common edge, the niobium atoms occupying the centres of the octahedra and being joined by a pair of bridging chlorine atoms. The Nb-Cl bridge bond-length is 2-56 Â ; the Nb-Cl non-bridging bond-lengths are 2-25 and 2-30 A (Fig. 2).

The X-ray powder photographs of TaClg are indistinguishable from those of NbCl5 except for differences in line intensities; the two com­

pounds may therefore be presumed to possess the same type of structure

THE HALIDES OF NIOBIUM AND TANTALUM 137

F I G . 2 . T h e s t r u c t u r e o f n i o b i u m ( V ) c h l o r i d e . ( Z a l k i n a n d S a n d s , 1 9 5 8 . )

and unit cell. The two pentachlorides form a complete series of mixed crystals.

It has been reported (Voitovich and Barabanova, 1961) that cryo- scopic measurements of the molecular weights of NbClg and TaClg in nitrobenzene give values which are close to the theoretical values for the monomers. On the other hand, NbClg has been found to remain as an undissociated dimer when dissolved in carbon tetrachloride or nitro­

methane; in acetonitrile the monomeric non-electrolyte adduct NbClg

CH3CN

is formed (Kepert and Nyholm, 1965). Similar results have been obtained for TaClg. I t is probable that the pentachlorides remain as dimers in non-complexing solvents and to a large extent also in the molten state; the latter is reflected in the high entropies of vaporization of the pentahalides. Also, Moureu et al. (1947) found that the Raman spectra of solid NbClg, solid TaClg, NbClg in carbon disulphide solution, and liquid TaClg were similar.

The mole-fractional solubility of tantalum(V) chloride in aromatic hydrocarbons, under rigorously anhydrous conditions, increases to a small but significant extent in the series: benzene < toluene < m- xylene < mesitylene, this increase in solubility being accompanied b y

T A B L E I V . W e i g h t s o l u b i l i t i e s i n h y d r o c a r b o n s

S o l v e n t S

( g/100 g s o l v e n t )

S o l v e n t S

( g/100 g s o l v e n t )

T a C l 5

C a r b o n t e t r a c h l o r i d e 0 - 6 4 T o l u e n e 1-05

C y c l o h e x a n e 0 - 5 0 m - X y l e n e 0 - 9 2

B e n z e n e 0 - 9 3 M e s i t y l e n e 0 - 9 5

N b C l g

B e n z e n e 1 0 3

138 Γ. FAIRBROTHER

Solid (155-183°) Solid (183-209°) Liquid (210-254°)

a n increase i n t h e d e p t h of colour of t h e solutions f r o m pale yellow t o orange. T h e solubilities of t a n t a l u m( V ) chloride i n cyclohexane a n d carbon tetrachloride, however, are less t h a n those i n t h e aromatic hydrocarbons, a n d t h e solutions are colourless. T h e solubility o f niobium(V) chloride i n d r y benzene is slightly greater t h a n t h a t o f t a n t a l u m( V ) chloride a n d t h e colour of t h e solution is red. T h e weight- solubilities of t a n t a l u m( V ) chloride i n the several aromatic hydrocarbons differ only t o a small e x t e n t ; t h e a p p r o x i m a t e solubilities (S) i n g halide/

100 g solvent a t 2 5 ° , interpolated f r o m t h e d a t a o f F a i r b r o t h e r et al.

(1965c), are given i n T a b l e I V .

A n u m b e r o f different values for t h e melting points have been p u b - hshed; t h e more recent figures are: NbClg, m.p. 2 0 4 · 7 - 2 0 9 · 5 ° ; TaClg, m.p. 2 1 6 - 5 - 2 2 0 ° (Schafer a n d P i e t r u c k , 1 9 5 1 ; Alexander a n d F a i r - brother, 1949a). T h e m . p . of NbClg is depressed b y traces of NbOClg;

M e y e r et al. (1961) f o u n d a eutectic a t 203-3° for about 5-5 mole % NbOClg.

M o l t e n niobium(V) chloride is a deep orange-red l i q u i d ; m o l t e n t a n t a l u m( V ) chloride is colourless.

T h e densities a n d viscosities of t h e melts i n each case are less t h a n those of t h e corresponding pentafiuorides. A t their respective melting points (i°), 4 - 2-0737 g c m - ^ for NbClg a n d 2-684 g c m - ^ for TaClg a n d 7 7= 0 - 9 2 1 a n d 1-003 centipoises respectively (Nisel'son a n d PustiFnik, 1963). T h e y are also poor conductors of electricity.

As i n t h e case o f t h e m e l t i n g points, a range o f values for t h e boiling points has been published: (a) a static m e t h o d , using a sickle gauge a n d niobium(V) chloride prepared f r o m spectrographically pure m e t a l , gave vapour pressures w h i c h m a y be represented b y

log ^ ( m m ) = 12-86 - 4 9 1 0 / T log ^ ( m m ) - 11-85 - 4 5 8 0 / y log p{mm) = 8-36 - 2890/T T h e last leads t o a boiling point of 254° (760 m m ) (Alexander a n d F a i r - brother, 1949a); (b) Schafer et al. (1952), f r o m a consideration of t h e results i n (a), a n d those of other workers w h o used different experi­

m e n t a l methods, conclude t h a t t h e vapour pressure of niobium(V) chloride can most probably be represented b y

Solid log :p(mm) = 11-5 - 4 3 7 0 / T L i q u i d log p(mm) = 8-37 - 2 8 7 0 / T the l a t t e r leading t o a boiling point (760 m m ) of 250°.

Corresponding t o these t w o sets o f figures are : heat o f sublimination NbCl5(s) (a) 2 2 - 5 - 2 0 - 9 kcal m o l e- i (two ranges); (b) 20-0 kcal m o l e- i :

T H E H A L I D E S O F N I O B I U M A N D T A N T A L U M 139

heat of volatiUzation N b C l s i l ) , (a) 13-2 kcal m o l e- i ; (b) 13-1 k c a l m o l e- i : entropy of volatiHzation of Hquid NbClg, (a) 25-0 cal deg-^; (b) 25-1 cal d e g- i .

T h e v a p o u r pressures o f t a n t a l u m ( V ) chloride h a v e also been mea­

sured b y d y n a m i c methods, b y estimations of t h e boiling points a t various pressures, a n d b y a static m e t h o d using a sickle gauge. T h e results o f t h e static measurements (Alexander a n d F a i r b r o t h e r , 1949a) m a y be expressed b y :

Solid (140-175°) : log p{mm) = 13-36 - 5 2 4 0 / Γ Solid (175-220°) : log p(mm) = 12-42 - 4 8 2 0 / T L i q u i d ( 2 2 0 - 2 4 0 ° ) : log ^ ( m m ) = 8-68 - 2 9 7 0 / T t h e last leading t o a boiling point (760 m m ) o f 239-3°.

There is a significant divergence f r o m l i n e a r i t y o f t h e log ρ against IIΤ relation for t h e solid, b u t no clearly defined b r e a k or visible poly­

morphic change. T h e heats o f sublimation for t h e t w o ranges are 24-0 a n d 22 kcal mole-^ respectively, w i t h a n average value of 2 3 - 0 kcal mole-^ over t h e whole range of solidity; t h e heat o f volatilization of TaCl5(l) is 13-6 kcal mole-^ a n d t h e e n t r o p y of v o l a t i l i z a t i o n o f t h e l i q u i d 26-5 cal deg-^.

A t all temperatures u p t o t h e boiling points, t a n t a l u m ( V ) chloride is more volatile t h a n t h e lighter n i o b i u m ( V ) chloride. T h e difference be­

t w e e n t h e v a p o u r pressures o f t h e t w o pentachlorides i n t h e solid state is small, b u t i n t h e l i q u i d state i t is sufficiently large t o p e r m i t o f t h e separation o f t h e pentachlorides b y fractional distillation (Steele a n d G e l d a r t , 1957).

I n t h e v a p o u r state, t h e pentachlorides are monomeric. This was first shown b y t h e classical w o r k o f D e v i l l e a n d Troost (1867) w h i c h served t o establish t h e separate identities o f n i o b i u m a n d t a n t a l u m , a n d has been confirmed i n t h e case of NbClg b y B a l k e a n d S m i t h (1908) a n d i n the case o f TaClg b y Schafer a n d Sibbing (1960). E l e c t r o n diffraction b y t h e vapours shows t h a t t h e molecules of t h e pentachlorides are trigonal b i p y r a m i d s , each m e t a l a t o m being surrounded b y five equi­

distant chlorine atoms; t h e N b —0 1 distance is 2-29 ± 0-03 Â , a n d t h e T a — C I distance 2-30 ± 0-02 A (Skinner a n d S u t t o n , 1940).

T h e heats o f f o r m a t i o n of t h e pentachlorides have been determined b y solution calorimetry: AH(NbClg) = —190-5 kcal mole-^ (Schafer a n d K a h l e n b e r g , 1960); AH^^s (TaClg) = - 2 0 5 - 0 k c a l m o l e- i (Schafer a n d K a h l e n b e r g , 1958). A n independent d e t e r m i n a t i o n o f Aifggs (NbClg)

= —190-6, a n d AH^^s (TaClg) = —205-5 k c a l mole-^ has also been m a d e b y Gross et al. (1960) b y t h e combustion of t h e metals i n chlorine.

140 F. FAIRBROTHER

(iii) Chemical Properties

T h e pentachlorides, like t h e pentafluorides are strongly electrophilic.

T h e i r reactions w i t h electron-donor species m a y be divided i n t o : r e ­ actions w i t h n e u t r a l ligands a n d reactions w i t h ionic ligands. T h e first group can be further sub-divided into reactions which lead t o t h e f o r m a ­ t i o n o f simple adducts, a d d i t i o n reactions w h i c h are accompanied or followed b y t h e elimination o f chlorine f r o m t h e pentachloride, a n d a d d i t i o n reactions accompanied or followed b y t h e reduction of t h e m e t a l t o a lower oxidation state.

I n reactions w i t h n e u t r a l ligands, t h e pentachlorides f o r m stable 1:1 complexes w i t h d i m e t h y l - , d i e t h y l - a n d di-n-propyl ethers a n d 1,4- d i o x a n ; t h e d i m e t h y l ether adducts sublime unchanged i n a v a c u u m a t about 100°, b u t t h e d i e t h y l ether a n d di-n-propyl ether adducts de­

compose below 100° t o give t h e m e t a l oxidetrichlorides a n d a l k y l chlorides (Cowley et al., 1958; Copley et al., 1964b; F e e n a n a n d Fowles, 1965). Stable complexes are also formed w i t h t h e d i - a l k y l sulphides, McgS, E t g S a n d ^^PrgS; all these sulphide complexes can be sublimed unchanged i n a v a c u u m . T h e r e is some evidence also of t h e f o r m a t i o n of TaCl5,2Me2S, w i t h a n incongruent m . p . a t about 15°.

T h e complexes formed w i t h d i e t h y l sulphide are t h e r m a l l y more stable t h a n those f o r m e d w i t h d i e t h y l ether (Fairbrother a n d N i x o n ,

1962). Also, d i e t h y l ether is readily displaced f r o m a complex b y d i e t h y l sulphide; moreover, a d i e t h y l sulphide is selectively absorbed b y nio- b i u m ( V ) chloride, w i t h t h e f o r m a t i o n of NbClg.EtgS, f r o m a gaseous m i x t u r e o f d i e t h y l sulphide a n d ether i n w h i c h t h e d i e t h y l sulphide is present only t o t h e e x t e n t of 1 0 % . D i m e t h y l ether, however, is n o t displaced b y d i m e t h y l sulphide f r o m its complex w i t h n i o b i u m ( V ) or t a n t a l u m ( V ) chloride, nor is t h e sulphide displaced b y t h e ether f r o m the complex TaClg.McaS, t h o u g h i n each case a t about —30° a q u a n t i t y of t h e second ligand is absorbed b y t h e complex i n question, b u t i t is completely evolved on evacuation a t room t e m p e r a t u r e , leaving t h e original complex. I n t h e case o f t h e complex NbCls.MegO t h e a m o u n t of d i m e t h y l sulphide absorbed a t l o w temperatures corresponds exactly to t h e f o r m a t i o n of a complex w i t h t h e composition NbCl5.Me2O.Me2S b u t t h e sulphide is evolved on evacuation a t a t e m p e r a t u r e well below r o o m t e m p e r a t u r e (Copley et al., 1964b).

T h e greater strength of t h e m e t a l - s u l p h u r bond as compared w i t h t h e m e t a l - o x y g e n b o n d i n t h e case o f t h e d i e t h y l ether a n d d i e t h y l sul­

phide complexes, is probably associated w i t h t h e greater polarizability of t h e sulphur a t o m a n d t h e greater dipole m o m e n t o f t h e sulphur- containing ligand. A further example of t h e same effect is found i n t h e t h i o x a n complexes, MCI5.C4H8OS, t h e infrared a n d nuclear magnetic

THE HALIDES OF NIOBIUM AND TANTALUM 141 resonance spectra o f which indicate t h a t t h e t h i o x a n bonds through

t h e sulphur a t o m a n d n o t t h r o u g h t h e oxygen a t o m ( F e e n a n a n d Fowles, 1965).

Complex f o r m a t i o n also takes place w i t h t h e cyclic thioethers t e t r a ­ hydrothiophen a n d pentamethylene sulphide. I t w o u l d appear t h a t t h e n a t u r e of t h e complexes f o r m e d w i t h these ligands depends u p o n t h e experimental conditions. F a i r b r o t h e r a n d N i x o n (1962), w o r k i n g w i t h excess o f u n d i l u t e d t e t r a h y d r o t h i o p h e n , obtained 1:2 complexes,

MCI5.2C4II8S,

w h i c h were insoluble i n excess of the ligand, though there was evidence f r o m t h e appearance o f t h e course o f t h e reaction t h a t this was a two-stage process, w i t h t h e f o r m a t i o n initially o f a soluble species. F e e n a n a n d Fowles (1965), using a threefold excess o f benzene as a diluent, obtained 1:1 complexes i n each case.

N i o b i u m ( V ) a n d t a n t a l u m ( V ) chlorides react vigorously w i t h phos­

phorus pentachloride t o f o r m ionic complexes:

[PCl4+][MCl6~].

These complexes do n o t m e l t on heating, b u t sublime like a m m o n i u m chloride ( G u t a n d Schwarzenbach, 1959). T h e y are soluble i n arsenic trichloride w i t h w h i c h t h e y f o r m t h e t e r n a r y complexes

PCI5.MCI5.ASCI3;

these can be crystallized f r o m solution i n

ASCI3.

W h e n arsenic trifluoride is added t o saturated solutions o f the complexes i n AsClg, tetrachloroniobium(V) fluoride or t e t r a c h l o r o t a n t a l u m ( V ) fluoride are formed ( K o l d i t z a n d F u r c h t , 1961; K o l d i t z et aL, 1964). I t has been suggested t h a t these m i x e d halides m a y be ionic i n t h e solid state. T h e y m e l t t o give homo- polar non-conducting liquids, b u t their electrical conductivities i n m e t h y l cyanide solution give evidence o f a n ionic structure; i n solvents o f weak polarity, a homopolar structure is favoured ( K o l d i t z et al,. 1964).

T h e pentachlorides also f o r m 1:1 complexes w i t h phosphorus o x y - chloride, t h e b o n d being f o r m e d t h r o u g h t h e oxygen a t o m ( G u t a n d Schwarzenbach, 1959). These complexes m e l t w i t h slight decomposi­

t i o n a t : NbClg.POCla, m . p . 124-5°; TaClg.POClg, m . p . 132-4°. T h e y are completely dissociated i n t h e vapour phase, b u t o n heating t h e l i q u i d complexes, relatively more

POCI3

is vaporized t h a n

MCI5

u n t i l i n each case a n azeotrope is formed, w i t h t h e compositions

NbCl5:POCl3

= 1-48:1, b.p. 263-0° a n d

TaCl5:POCl3

= 1-15:1, b.p. 285-7°. These azeotropes have been suggested as media for t h e separation o f t h e pentachlorides b y fractional distillation (Nisel'son, 1957; Sheka et al., 1959).

T h e crystal structure of NbCl5.POCl3, w h i c h has been determined b y single crystal methods (Braenden a n d L i n d q v i s t , 1963), conflrms t h a t t h e t w o components are bonded t h r o u g h t h e oxygen a t o m . T h e hygro­

scopic crystals, which were prepared i n sealed tubes, are orthorhombic, w i t h a = 8-07 Â ; δ = 16-23 A; c = 8-83 Â .

142 Γ. FAIRBROTHER

B y contrast w i t h t h e thioethers, there is no noticeable interaction be­

t w e e n these pentachlorides a n d thiophosphoryl chloride, f r o m a solu­

t i o n i n w h i c h t h e y can be recovered unchanged (Nisel'son, 1960). I t m a y be n o t e d i n this connection t h a t whereas t h e dipole m o m e n t o f d i e t h y l sulphide (1-58 D ) is greater t h a n t h a t o f d i e t h y l ether (1-15 D ) , i n t h e case of t h e phosphoryl compounds t h e reverse is t h e case

(PSCI3, 1-41 D ; POCI3, 2-39 D ) .

Triphenylphosphine oxide a n d hexamethylphosphoramide f o r m 1:1 complexes w i t h n i o b i u m ( V ) chloride t h a t can be obtained as crystals f r o m a solution o f equimolar amounts o f pentachloride a n d ligand i n a n anhydrous m e t h y l c y a n i d e - m e t h y l e n e chloride m i x t u r e . Similar com­

pounds are formed b y t a n t a l u m ( V ) chloride w i t h triphenylphosphine oxide a n d diphenylbenzylphosphine oxide. T h e infrared spectra of these compounds indicate a large decrease i n each case o f the Ρ = 0 stretching frequency, w h i c h results f r o m t h e great affinity o f t h e m e t a l atoms for oxygen ( B r o w n et ah, 1966).

T h e addition o f o-phenylenebisdimethylarsine (Diarsine) t o solutions of the pentachlorides i n d r y non-hydroxylic solvents leads t o t h e f o r m a ­ t i o n of 7 co-ordinate 1:1 complexes o f t h e t y p e MCI5.Diarsine. These compounds are monomeric i n benzene solution a n d are non-conducting i n d r y n i t r o m e t h a n e or acetonitrile (Clark et al., 1965b).

T h e preparation o f NbCl5.2Me3N, TaCl5.2Me3N, a n d TaCl5.Et3N has been reported (Fowles a n d Pleass, 1957; Carnell a n d Fowles, 1959) b u t the usual reactions o f nitrogen bases lead t o t h e elimination o f chlorine or reduction o f t h e m e t a l t o a lower o x i d a t i o n state. These reactions are discussed below.

T h e pentachlorides react w i t h a l k y l cyanides R C N , (R = M e , E t ,

^Pr) t o give crystalline 1:1 adducts, MCI5.RCN. These compounds are monomeric i n benzene solution, are diamagnetic a n d are non-electro­

lytes w h e n dissolved i n excess a l k y l cyanide (Feenan a n d Fowles, 1964).

I n chlorine-elimination reactions, although 1:1 complexes are f o r m e d w i t h triphenylphosphine oxide w h e n equimolar proportions of t h e r e ­ actants are used ( B r o w n et al., 1966), t h e pentachlorides react w i t h excess of triphenylphosphine oxide t o give oxidechlorocomplexes of t h e t y p e MOCl3.2(C6H5)3PO. I n a similar manner, t h e products NbOCl3.

2(C6ll5)3AsO a n d TaOCl3.2(C6H5)3AsO are obtained w h e n t h e penta­

chlorides are t r e a t e d w i t h excess o f triphenylarsine oxide (Copley et al., 1965). B y contrast, hexamethylphosphoramide forms only t h e 1:1 com­

plex, even i n t h e presence of excess of ligand ( B r o w n et al., 1966).

T h e presumed i n i t i a l f o r m a t i o n of adducts between the pentachlorides a n d h y d r o x y l compounds, e.g. alcohols, phenols, carboxylic acids etc., is usually followed i m m e d i a t e l y b y elimination o f hydrogen chloride a n d

T H E H A L I D E S O F N I O B I U M A N D T A N T A L U M 143

t h e f o r m a t i o n of a n M — 0 — R g r o u p ; t h i s r e a c t i o n m a y b e r e p e a t e d , t h e n u m b e r of chlorine a t o m s w h i c h a r e displaced from t h e central m e t a l a t o m d e p e n d i n g u p o n t h e c i r c u m s t a n c e s of t h e reaction. T h u s , m o l t e n phenol r e a c t s w i t h t h e p e n t a c h l o r i d e s t o give M(OC6H5)5, a n d j9-naphthol w h e n refluxed w i t h t h e p e n t a c h l o r i d e s in c a r b o n t e t r a ­ chloride suspension, gives M(OCioH7)5 ( F u n k a n d B a u m a n n , 1937), whilst u n d e r milder conditions, e.g. i n C S g solution, M ( 0 C 6 H 4 ) 4 C 1 or M(OCioH7)3Cl2 m a y b e o b t a i n e d , a n d w i t h a n t h r a n o l , M(OCi4H9)2Cl3 ( F u n k a n d M e d l a n d e r , 1928). Similarly, w h e n t h e p e n t a c h l o r i d e s a r e dissolved in d r y alcohol, only p a r t of t h e chlorine is a t first displaced, w i t h t h e f o r m a t i o n of MCl2(OEt)3; w h e n d r y a m m o n i a gas is passed t h r o u g h t h e solution, h o w e v e r , t h e alcoholysis goes t o completion, w i t h t h e p r e c i p i t a t i o n of a m m o n i u m chloride a n d t h e f o r m a t i o n of t h e p e n t a - e t h o x i d e in solution, from w h i c h it m a y b e o b t a i n e d b y e v a p o r a t i o n . I n t h i s w a y , t h e n o r m a l p e n t a - a l k o x i d e s M ( 0 R) 5 , (R = Me, E t , ^^Pr,

^Bn, a n d ^^pentyl) h a v e b e e n p r e p a r e d (Bradley et al., 1955, 1956).

I t h a s also b e e n r e p o r t e d t h a t w h e n t h e p e n t a c h l o r i d e s a r e m e l t e d w i t h a n excess of salicylic acid a n d t h e u n c h a n g e d ligand e x t r a c t e d w i t h ether, t h e c o m p o u n d s M2(OC6H4COO)5 a r e o b t a i n e d ; if t h e p e n t a ­ chlorides a r e t r e a t e d w i t h a n ethereal solution of t h e salicylic acid, h o w ­ ever, only t h r e e of t h e five chlorine a t o m s a r e replaced, giving Nb2Cl4(OC6H4COO)3. Also, if catechol is slowly a d d e d in excess t o solu­

t i o n s of t h e p e n t a c h l o r i d e s in boiling b e n z e n e , t h e c o m p o u n d s M ( O C 6 H 4 0 H) 5 are o b t a i n e d ; w i t h a catechol t o p e n t a c h l o r i d e r a t i o of 2-5:1, t h e c o m p o u n d Nb2(C6ll402)5 w a s o b t a i n e d ( F u n k et al., 1959).

N o s t r u c t u r e s h a v e b e e n given for t h e s e c o m p o u n d s .

Complexes of t h e t y p e MCl2AcAc(OMe)2 a n d MCl2AcAc(OEt)2 h a v e b e e n o b t a i n e d b y t h e r e a c t i o n of t h e p e n t a c h l o r i d e s w i t h a c e t y l a c e t o n e (AcAc) in m e t h a n o l or e t h a n o l solution respectively (Rosenheim a n d R o e h r i c h , 1932; Djordjevic a n d K a t o v i c , 1963).

Following u p o n t h e e s t a b l i s h m e n t of a p H scale in a b s o l u t e m e t h a n o l , t h e formation of m e t h o x i d e d e r i v a t i v e s of niobium(V) a n d t a n t a l u m ( V ) chlorides h a s b e e n s t u d i e d q u a n t i t a t i v e l y ; t h i s p r o c e d u r e consists of t h e t i t r a t i o n of t h e p e n t a c h l o r i d e s b y l i t h i u m m e t h o x i d e in a n h y d r o u s m e t h a n o l c o n t a i n i n g chloride ion (Gut, 1964). T h e s a m e t e c h n i q u e h a s b e e n u s e d t o s t u d y , q u a n t i t a t i v e l y , t h e f o r m a t i o n of a c e t y l a c e t o n e a n d catechol complexes in m e t h a n o l solution. A m o n g t h e n e w complexes p r e p a r e d d u r i n g t h i s w o r k were t h e complexes: (Nb/Ta)AcAc(0Me)3Cl a n d (Nb/Ta)(0Me)4Cl (Gut et al, 1965).

T h e r e p l a c e m e n t of t w o a t o m s of chlorine b y a n o x y g e n a t o m , a c c o m p a n i e d b y t h e co-ordination of a d d i t i o n a l molecules of ligand, such as occurs w h e n t h e p e n t a c h l o r i d e s a r e t r e a t e d w i t h a n excess of