• Nem Talált Eredményt

IT IS INVOLATILE AND, AS MENTIONED ABOVE, DISPROPORTIONATES ON HEAT

B. Sub-bromides (i) Tetrabromides

Niobium(IV) bromide may be prepared, in much the same manner as niobium(IV) chloride, by reduction of the pentabromide with nio­

bium metal in a sealed tube in a temperature gradient of 300° (Nb end) to 200° (NbBrg end) (Schafer and Dohmann, 1961) or 410° to 350°

(McCarley and Torp, 1963). The NbBr4 is deposited as crystals in the middle zone of the tube. The conditions necessary for this reaction to give the required product are limited on the one hand by the minimum temperature necessary for the reduction to occur, and on the other hand by the disproportionation of the niobium(IV) bromide at higher tem­

peratures; for example, changing the temperature of the niobium metal from 410-450° was sufficient to produce a lower bromide rather than NbBr4 (McCarley and Torp, 1963).

The X-ray powder pattern of N b B r 4 has been indexed on the basis of an orthorhombic unit cell containing four NbBr4 units and with a = 8-60 A; b = 9-31 Â ; c = 7-19 Â. These values give a calculated density of 4-77 g cm-^, which is in good agreement with the observed pycnometric density of 4-65 g cm-^. The X-ray powder photograph is so closely similar to that of NbCl4, that one may assume that they have the same crystal structure; the solid also is diamagnetic. Niobium(IV) bromide is extremely sensitive to moisture. I t dissolves in dilute hydro­

chloric acid to give a solution with a deep blue colour, which fades on exposure to the air as the niobium(IV) is oxidized to the Nb(V) state.

THE HALIDES OF NIOBIUM AND TANTALUM 165

Tantalum(IV) bromide was first prepared by Gutmann and Tannen-berger (1956) by the reduction of tantalum(V) bromide vapour by hydrogen in an electrodeless discharge. It may be prepared by the re­

duction of tantalum(V) bromide vapour by tantalum metal in a simple closed tube which is maintained in a temperature gradient (Berdonosov et al., 1962; McCarley and Boatman, 1963). Considerations similar to those which apply to the preparation of niobium(IV) bromide, limit the temperature of the "cool'' end of the reaction tube to about 330°; above this temperature, lower bromides are formed. The reduction of tanta-lum(V) bromide vapour by tantalum has, more recently, also been studied by the ' 'three-temperature" technique which also serves to obtain information regarding the regions of stability of TaBr4 and of lower bromides (Schafer et al., 1965a). For the preparation of TaBr4, the appropriate conditions are: TQ (TaBr5(l) = 300°; (TaBr4(s)) = 310°;

T2 (Ta(s)) = 620°. Tantalum(IV) bromide has also been prepared by the reduction of TaBrg vapour which is in equilibrium with liquid TaBrg at 250° (pressure --60 mm), by aluminium foil at 500° (McCarley and Boatman, 1963).

Tantalum(IV) bromide forms large brown-black crystals which are diamagnetic and probably contain T a — T a pairs. It is isomorphous with niobium(IV) bromide and has been indexed on the basis of an ortho-rhombic unit cell containing four TaBr4 units and with a ==9-58 Â ; b = 9-30 A ; c = 7-21 Â (McCarley and Boatman, 1963).

(ii) Tribromides and Lower Bromides

A compound with the composition NbBrg was obtained by Brubaker and Young (1951) by the action of hydrogen on niobium(V) bromide vapour at 500°. It has been found, however, that this compound, like the trichloride, exists as a homogeneous phase over a range of composi­

tions which extends from NbgBrg to about NbBrg.ga (Schafer and Dohmann, 1961). It can be prepared, in a similar manner to ''NbClg", by the reaction of niobium metal with niobium(V) bromide vapour in a temperature gradient of 450° (Nb) to 400° (NbBrg). The composition of the resulting homogeneous phase can be related to the excess of nio-bium(V) bromide which has been used. Crystals of the ''tribromide", which are several millimetres in length, can be obtained in this way.

Within the range of homogeneity of the crystals, the reactivity under­

goes a change: bromine-rich crystals ( /^NbBrg) are sensitive to moisture, whilst near the lower end of the phase-range ( '^NbBrg.?) the crystals can be handled in the air.

Tantalum tribromide is formed by the disproportionation of TaBr4 at temperatures above 300° (Gutmann and Tannenberger, 1956). It is

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

also formed as an intermediate product during the reduction of tan-talum(V) bromide vapour b y hydrogen at 700° (Young and Hastings, 1942) b y the reaction of hydrogen with the solid bromide at 135-200°

(Berdonosov et al., 1962), and b y the reaction of tantalum metal with hydrogen bromide at temperatures of 500° and above (Young and Brubaker, 1952). I t is most conveniently prepared, however, b y the

•'three temperature" method as used for the preparation of tantalum (IV) bromide (Schafer et al., 1965a); the tantalum is maintained at 620°, the TaBrg at 320° and the tantalum tribromide deposited in a middle zone at 380°. B y varying the temperatures of the three zones, especially those of the TaBrg and the middle condensation zone, the ''TaBrg", like the other trihalides, can be obtained as a homogeneous phase over a range of com­

positions. A preparation with the composition TaBrg-go showed a mag­

netic susceptibility of Xg X 10^ = - f O - 5 6 , + 0 - 1 2 a n d —0-03e.m.u, at 90, 195 and 297 °K respectively ; a preparation with the composition TaBrg. ^^j, however, was diamagnetic at all three temperatures, withXg χ 10^= —0-1 e.m.u. Tantalum tribromide, like tantalum trichloride, has a graphite­

like feel on rubbing, which is indicative of the existence of a layer lattice.

The pycnometric density (TaBrg.go) df=^'2ZgGmr^ (Schafer et αί.,1965a).

I t has also been reported (Schafer et al., 1965a) that at still higher temperatures, (TaBrs) = 330°; (TaBrg.g) = 450°; (Ta) = 620°, a new phase with the composition TaBrg.g is obtained. This is a black, crystalline paramagnetic substance with a susceptibility Xg χ 10^

= + 0 - 4 2 , + 0 - 3 3 , + 0- 2 6 e.m.u. at 90, 195 and 296°K respectively.

Bromotantalum bromide, Ta6Bri4.7H20 was prepared b y Chapin (1910) who measured its molecular weight ebullioscopically (in propyl alcohol) and cryoscopically (in water) and showed that only y of the bromine was ionic in aqueous solution. Chapin used sodium amalgam as the reducing agent, but a better yield is obtained b y the use of granulated lead, which is mixed with the pentabromide and heated under a cover of nitrogen (Lindner and Feit, 1924). The preparation of the corresponding niobium compound has proved to be more difficult;

Harned (1913) was unable to obtain it b y sodium amalgam reduction of the pentabromide, but it has been reported (Harned et al., 1960) that fair yields m a y be obtained by the use of cadmium.

X - R a y diffraction studies of ethanol solutions of Ta6Bri4.7H20 (Vaughan et al., 1950) show that it possesses the same basic structure of Tag octahedra as chlorotantalum chloride, Ta6Cli4.7H20. The prepara­

tion of the anhydrous Ta6Bri4, or the examination of its structure b y X-ray crystal methods, have not y e t been reported, but since Ta6li4 and Nb6Cli4 are stated to be isotypical (Bauer et al., 1965), it is reasonable to suppose that the structure of TagBri4 will conform to the same pattern.

THE HALIDES OF NIOBIUM AND TANTALUM 167

4. Iodine Compounds A. Pentaiodides

(i) Preparation and Physical Properties

T h e p e n t a i o d i d e s p r e s e n t a c o n t r a s t i n t h e i r m e t h o d s of p r e p a r a t i o n a n d properties, b o t h from t h e o t h e r p e n t a h a h d e s a n d from one a n o t h e r . Niobium(V) iodide c a n n o t b e p r e p a r e d or sublimed e x c e p t in t h e presence of a n excess pressure of iodine v a p o u r ; t a n t a l u m ( V ) iodide on t h e o t h e r h a n d is stable u p t o a t least 500-600° a n d can b e sublimed i n a v a c u u m . T h e loss of iodine b y niobium(V) iodide o n h e a t i n g in a v a c u u m c a n b e observed even a t a b o u t 200°, a n d t h e decomposition t o t h e t e t r a i o d i d e in these circumstances is complete a t 270° (Corbett a n d S e a b a u g h , 1958), where t h e s u b l i m a t i o n pressure of t h e p e n t a i o d i d e c a n n o t be m o r e t h a n a few millimetres.

Niobium(V) iodide can be p r e p a r e d b y t h e action of a n h y d r o u s h y d r o g e n iodide on t h e v a p o u r of t h e p e n t a b r o m i d e , b u t t h e r e a c t i o n is slow a n d i n c o m p l e t e ; it is m u c h m o r e conveniently p r e p a r e d b y t h e action of iodine v a p o u r on t h e m e t a l in a sealed t u b e . T h e p e n t a i o d i d e begins t o form a t a b o u t 250° a n d forms r a p i d l y a t a b o u t 280°; t h e entire t u b e a n d i t s c o n t e n t s a r e m a i n t a i n e d a t t h e s a m e t e m p e r a t u r e (Rolsten, 1957). E x c e s s iodine can b e r e m o v e d b y s u b l i m a t i o n a t a t e m p e r a t u r e a b o u t its m e l t i n g p o i n t (114°); t h e o p e r a t i o n is a slow one on a c c o u n t of t h e low t e m p e r a t u r e level r e q u i r e d t o p r e v e n t t h e r m a l decomposition of t h e pentaiodide, or t h e r e d u c t i o n of t h e p e n t a i o d i d e b y residual n i o b i u m m e t a l . W h e n p r e p a r e d i n t h i s w a y , niobium(V) iodide m a y b e o b t a i n e d as crystals several m m in l e n g t h a n d h a v i n g t h e a p p e a r a n c e of brass filings (Alexander a n d F a i r b r o t h e r , 1949b) or as brass-coloured p l a t e s (Rolsten, 1957), which are i m m e d i a t e l y h y d r o l y s e d on exposure t o t h e air. N o single-crystal X - r a y diffraction d a t a are re­

corded, b u t only t h e ci-spacings for t h e p o w d e r e d crystals (Rolsten, 1957).

As a consequence of its t h e r m a l instability, n o m e a s u r e m e n t s h a v e b e e n m a d e of t h e v a p o u r pressure of niobium(V) iodide, t h o u g h t h i s should n o t b e impossible b y t e c h n i q u e s similar t o t h o s e used for o t h e r u n s t a b l e halogen c o m p o u n d s ; b y a n a l o g y w i t h t h e o t h e r p e n t a h a l i d e s , however, i t m a y b e a s s u m e d t o b e less t h a n t h a t of t a n t a l u m ( V ) iodide.

T a n t a l u m ( V ) iodide also m a y b e formed b y t h e a c t i o n of h y d r o g e n iodide on t h e v a p o u r of t h e p e n t a b r o m i d e , b u t is m o r e conveniently p r e p a r e d b y t h e action of iodine v a p o u r on t h e h e a t e d m e t a l ; in a closed t u b e , t h e r e a c t i o n begins t o t a k e place a t a b o u t 300° b u t occurs m o r e r a p i d l y a t 340-370° (Rolsten, 1958b). Owing t o t h e m u c h g r e a t e r t h e r ­ m a l stability of t a n t a l u m ( V ) iodide, however, m u c h higher t e m p e r a t u r e s a n d r a t e s of reaction m a y be used. T a n t a l u m ( V ) iodide m a y be purified b y s u b l i m a t i o n in a v a c u u m a t a b o u t 500°; t h e r e is n o evidence of a n y

168 F. FAIRBROTHER

d i s s o c i a t i o n , o r r e d u c t i o n b y t h e m e t a l , b e l o w i t s b o i l i n g p o i n t ( 5 4 3 ° / 7 6 0 m m ) . T a n t a l u m ( V ) i o d i d e i s u s u a l l y o b t a i n e d a s a m i c r o c r y s t a l l i n e b r o w n p o w d e r f r o m w h i c h s m a l l b l a c k c r y s t a l s m a y b e o b t a i n e d b y s l o w s u b l i m a t i o n ; t h e s e c r y s t a l s p o s s e s s a m e t a l h c l u s t r e .

X - R a y p o w d e r a n d s i n g l e c r y s t a l m e t h o d s s h o w t h a t t h e s t r u c t u r e c o n s i s t s o f a n o r t h o r h o m b i c u n i t c e l l c o n t a i n i n g e i g h t T a l g u n i t s a n d w i t h a = 6 - 6 5 A; δ = 1 3 - 9 5 A; c 2 0 - 1 0 A. T h e X - r a y d e n s i t y i s 5 - 8 0 9 a n d t h e p y c n o m e t r i c ( i f = 5 - 7 9 8 ± 0 - 0 0 8 g c m - ^ ( R o l s t e n , 1 9 5 8 b ) . T a n t a l u m ( V ) i o d i d e i s s t a t e d n o t t o b e i s o s t r u c t u r a l w i t h n i o b i u m ( V ) i o d i d e .

T h e v a p o u r p r e s s u r e s o f s o l i d t a n t a l u m ( V ) i o d i d e f r o m 3 8 0 - 4 9 0 ° , a n d o f t h e l i q u i d f r o m 5 0 0 - 5 4 0 ° ( A l e x a n d e r a n d F a i r b r o t h e r , 1 9 4 9 b ) a r e g i v e n b y :

( s o l i d ) : l o g p ( m m ) = 8 - 2 1 - 4 3 0 0 / Γ ( h q u i d ) : l o g p ( m m ) = 7 - 6 7 - 3 9 5 0 / T

f r o m w h i c h t h e h e a t s o f s u b l i m a t i o n a n d v o l a t i l i z a t i o n a r e 1 9 - 7 k c a l m o l e - i a n d 1 8 - 1 k c a l m o l e - ^ r e s p e c t i v e l y , a n d t h e h e a t o f f u s i o n ( b y d i f f e r e n c e ) 1 - 6 k c a l m o l e ~ ^ . T h i s s m a l l h e a t o f f u s i o n m a k e s t h e t r i p l e p o i n t d i f f i c u l t t o o b s e r v e , b u t e s t i m a t e s f r o m b o t h a l a r g e s c a l e p-t c u r v e a n d t h e l o g p-ljT p l o t s , i n d i c a t e a t r i p l e p o i n t o f 4 9 6 ± 2 ° ; t h e b o i l i n g p o i n t i s n e a r t h e s o f t e n i n g t e m p e r a t u r e o f P y r e x g l a s s , b u t a s h o r t e x t r a p o l a t i o n o f t h e p-t c u r v e g i v e s t h e b . p . a t 7 6 0 m m a s 5 4 3 ± 0 - 5 ° .

(ii) Chemical Properties

N i o b i u m ( V ) i o d i d e i s t h e m o s t s e n s i t i v e t o t h e a i r o f a l l t h e h a l i d e s o f t h e s e m e t a l s ; o n e x p o s u r e t o t h e a i r t h e b r a s s c o l o u r e d c r y s t a l s i m m e d i a t e l y l o s e t h e i r l u s t r e a n d h y d r o g e n i o d i d e i s e v o l v e d . O n a c c o u n t o f t h e d i f f i c u l t y o f p r e p a r i n g n i o b i u m ( V ) i o d i d e i n a p u r e c o n ­ d i t i o n , l i t t l e a t t e n t i o n h a s b e e n p a i d t o t h e p o s s i b i l i t y o f i t s f o r m i n g s i m p l e a d d u c t s w i t h o u t d e c o m p o s i t i o n . W i t h e x c e s s o f p y r i d i n e i t d i s s o c i a t e s , b y c o n t r a s t w i t h t h e p e n t a c h l o r i d e s a n d p e n t a b r o m i d e s w h i c h a r e r e d u c e d , t o g i v e N b l 4 . 2 p y a n d t h e p y r i d i n e a d d u c t o f m o l e ­ c u l a r i o d i n e ( M c C a r l e y et al., 1 9 6 3 ) . o - P h e n y l e n e ( b i s d i m e t h y l ) a r s i n e ( D i ­ a r s i n e ) r e a c t s w i t h N b i g , w h e n h e a t e d i n a s e a l e d t u b e a t 7 0 ° i n a m o l e -r a t i o o f 3 : 1 t o g i v e t h e y e l l o w c o m p l e x , N b l 4 . 2 D i a -r s i n e . T h i s c o m p l e x i s n o t i s o m o r p h o u s w i t h t h e c o r r e s p o n d i n g c h l o r o - a n d b r o m o - c o m p l e x e s , t h o u g h e x a m i n a t i o n o f t h e v i s i b l e s p e c t r a o f t h e t h r e e c o m p l e x e s i n d i ­ c a t e s t h a t i n e a c h c a s e t h e m e t a l h a s t h e s a m e s t e r e o c h e m i s t r y ( C l a r k et al., 1 9 6 5 a ) . W h e t h e r t h i s r e a c t i o n , l i k e t h e p y r i d i n e r e a c t i o n , i n v o l v e s t h e r e m o v a l o f m o l e c u l a r i o d i n e a s a c o m p l e x f r o m t h e p r e s u m e d N b g l x o d i m e r , o r i s a n o x i d a t i o n - r e d u c t i o n r e a c t i o n i n v o l v i n g o x i d a t i o n p r o ­ d u c t s o f d i a r s i n e , a p p a r e n t l y r e m a i n s t o b e d e t e r m i n e 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 169

Tantalum(V) iodide, b y contrast with the pentachloride, gives no evidence of complex formation with diethyl ether, in which it dissolves only to the extent of 3-27 g per 100 g ether to give a red-brown solution from which the Talg can be recovered unchanged by evaporation (Cowley et ah, 1958). On the other hand, vapour pressure-phase studies indicate the formation of a 1:1 complex with diethyl sulphide, Talg.EtaS, which is moderately stable in a vacuum up to 200°, but at higher temperatures decomposes into Talg and diethyl sulphide. Tantalum(V) iodide also dissolves in tetrahydrothiophen, from which on concentration a brown-black solid of ill-defined composition, possibly Tal5.2C4H8S, together with excess of ligand, was obtained (Fairbrother and Nixon, 1962). This complex decomposed in a vacuum at 150°, with evolution of iodine.

Tantalum(V) iodide reacts with pyridine in the same manner as niobium(V) iodide, the complexes Tal4.2py and py.l2 being formed; the reaction, however, appears to be less complete, which is in accord with the much greater stability of tantalum(V) iodide with respect to the thermal dissociation:

MUs) = Ml4(s) + iUg) (McCarley and Boatman, 1963).

No reduction of tantalum(V) iodide by Diarsine was observed.

Niobium oxidetri-iodide NbOIg has been prepared b y the reaction of niobium metal, iodine and niobium pentoxide in a sealed tube in a temperature gradient (Schafer and Gerken, 1962).

3Nb 4 - 7-512 + Nb205 = 5 N b O l 3

The reactants—the iodine and pentoxide being in excess—are heated together to 400° in one half of a 20 cm long evacuated tube, the other end of which is maintained at 275°. After 2 days a good yield of black needles, up to 8 mm in length, collects in the 275° zone. X - R a y diffrac­

tion values have been recorded, but so far not indexed. The compound possesses a diamagnetism which is almost independent of temperature (XG X 1 0 6 = —0-24 e.m.u. at 9 0 , 1 9 5 and 296°K). I t is easily hydrolysed on exposure to the air; the intermediate product of hydrolysis, N b 0 2 l is stated to have been isolated (Schafer et al., 1964a).

On heating in the absence of air, e.g. in an argon atmosphere, NbOIs decomposes to give N b O l 2 :

2 NbOl3(s) = 2 NbOl2(s) + 12(g)

The iodine pressure of this decomposition reaches 1 atm at 270°. In a closed tube, NbOIg can be subhmed.

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

Β. Sub-iodides (i) Tetraiodides

(a) Niobium(IV) iodide. Reference has been made to the ease with which niobium(V) iodide loses iodine. Niobium(IV) iodide may conveniently be prepared by heating niobium(V) iodide to 270° for 48 h at a pressure of about 0-8 mm. If the decomposition temperature is raised to 300°, the tetraiodide is obtained as a crystalline sublimate in the form of elongated hexagonal plates and fine needles; at 430° disproportionation into the pentaiodide and involatile tri-iodide takes place (Corbett and Seabaugh, 1958).

A n X-ray crystal study of the 'low-temperature" or α-form of N b l 4 which is obtained in this way, has shown that the crystals are ortho-rhombic, with space group Cmc2-^ and a = 1-67 A, b == 13-23 A;

c = 13-93

A,

the unit cell containing eight N b l 4 units (Dahl and Wampler, 1962). The structure is based on a distorted hexagonal closest packed array of iodine atoms, with J of the octahedral holes occupied by niobium atoms in such a way as to form infinite chains, parallel to the α-axis, of Nbig octahedra sharing two opposite edges. The niobium atoms in these octahedra are shifted from the centres in alternate direc­

tions so as to give pairs of niobium atoms at a distance of Nb—^Nb

= 3-31 A. This structure, in conjunction with the diamagnetism of the crystals, leaves little doubt that metal-metal bonding occurs between the pairs of niobium atoms in this case also.

Niobium(IV) iodide appears to be trimorphic (Seabaugh and Corbett, 1965). In addition to the ''low-temperature" or α-form, a weakly endo-thermic a ^ ή transition at about 348° has been observed by thermal methods but is without effect on the X-ray powder pattern. A further transition occurs at 417°, to γ N b l 4 , which melts incongruently at 503°

to give an iodine-rich liquid and solid Nbglg. The structure of y- N b l 4 is derived from the jS-TiClg (ZrXg) structure, with the 1:4 metal to iodine atom ratio accommodated by disordering the metal atoms over three-quarters of the usual cation sites (Seabaugh and Dahl, in Seabaugh and Corbett, 1965).

(b) Tantalum(IV) iodide. Tantalum(V) iodide, by contrast with nio-bium(V) iodide, is thermally stable at least to the softening temperature of Pyrex glass, but the reduction of its vapour by metallic tantalum at any temperature at which tantalum(IV) iodide is stable, is so slow as to make this method of preparation impracticable. Tantalum(V) iodide may, however, be reduced by aluminium foil in a sealed tube in a tem­

perature gradient of 500° (Al) to 350° (Talg) (McCarley and Boatman, 1963). This reaction also is slow, since the vapour pressure of Talg at 350° is only about 16 mm, and 7 days are required to obtain a yield of

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 171

5 g. A n alternative method of preparation consists in heating the pyri­

dine adduct, Tal4.2py, which is formed when tantalum(V) iodide is shaken with excess pyridine for 3 days at room temperature in the absence of air, to 200° for 2 days under a vacuum. The yield is almost quantitative. Tantalum(IV) iodide apparently exists in two forms, according to whether it is prepared b y the high-temperature reduction of Talg vapour b y aluminium, or b y the thermal decomposition of the pyridine adduct. These two forms have different X-ray powder pat­

terns, neither of which can be indexed on the orthorhombic cell and space group of Nbl4. I t has been suggested, however (McCarley and Boatman, 1963), that the two forms m a y correspond qualitatively to two of the N b l 4 polymorphs, viz. (i) the Tal4 which is formed b y the thermal decomposition of the pyridine adduct, corresponding to a-Nbl4 and (ii) that which is deposited at higher temperatures following the reduction of Talg vapour b y aluminium, corresponding to one of the high-temperature modifications of Nbl4.

The only tetraiodide adducts that have been reported up to the pre­

sent are the pyridine adducts and the niobium(IV) iodide-Diarsine adduct already mentioned.

(c) M(IV) Oxideiodide, NbOIg is formed as a residue when NbOIg is heated in a vacuum (p. 169). I t may be prepared b y a similar technique to that used for the preparation of NbOIg, but with the difference that niobium metal and iodine are used in the stoichiometric proportions required b y the equation:

3 N b + NbgOg + I 2 - 5 N b O l 2

together with an excess of NbgOg. The reactants are heated together at 500° and the NbOIg collected in almost quantitative yield at 450°.

The involatile NbOIg is transported thermally along the tube as a result of the small amount of NbOIg also formed and the reversibility of the reaction:

2 N b O l 2( s ) + 12(g) <± 2NbOl3(g)

The black prismatic crystals of NbOIg are very stable towards air and aqueous mineral acids, and are diamagnetic (Xg = —0-21 χ 10~^ e.m.u.

at 90, 195 and 296°K). They decompose when heated alone above 500°, the final residue being a mixture of NbO and NbOg, the proportion of NbO increasing with the temperature of decomposition (Schafer and Gerken, 1962).

(ii) Tri-iodides and Lower Iodides

(a) Niobium tri-iodide is obtained when niobium(V) iodide or niobium(IV) iodide are heated at 425-430° for 48 h under a vacuum, the evolved

172 F. FAIRBBOTHER

iodine being condensed (Corbett a n d Seabaugh, 1958) or as a subhmate f r o m t h e pyrolysis of lower iodides. I t is also obtained as t h e principal initial product of t h e reaction of higher iodides w i t h excess n i o b i u m m e t a l i n a sealed t u b e u p t o '-'500°, although t h e phases i n equilibrium under these conditions w o u l d be expected t o consist o f Nbgig a n d u n -reacted m e t a l .

N i o b i u m tri-iodide begins t o decompose a t about 5 1 3 ° , w i t h little evidence of reversibility, i n t o t h e involatile NbgIg a n d a n iodine-rich l i q u i d w h i c h can be sublimed a w a y .

(b) Triniohium octaiodide NbgIg, w h i c h is obtained i n this w a y b y t h e t h e r m a l decomposition of higher iodides, is a black a n d v e r y stable compound. I t is stable i n air over a period of several days a n d is only slowly a t t a c k e d b y h o t H C l or dilute H N O g . Q u a l i t a t i v e measurements show t h a t i t is diamagnetic (Seabaugh a n d Corbett, 1965).

N i o b i u m tri-iodide a n d t r i n i o b i u m octaiodide present a contrast t o the corresponding chlorides. I t has been observed t h a t " n i o b i u m t r i ­ chloride" a n d " n i o b i u m t r i b r o m i d e " are i n effect only single stages i n rather wide ranges o f solid solutions, w i t h NbgClg a n d NbgBrg as t h e lower limits. B y contrast, N b i g appears t o be a quite separate stoichio­

metric compound; there is no significant v a r i a b i l i t y of composition t o be observed, either of N b I g or NbgIg or a n y evidence of a solid solution range between t h e m . T h e phase d i a g r a m of t h e condensed N b l g - N b system, between about 300 a n d 600° has been given b y Seabaugh a n d Corbett (1965).

(c) Tantalum tri-iodide has only been mentioned briefiy, a n d its charac­

terization is still uncertain. McCarley a n d B o a t m a n (1963), during their study of the preparation of t a n t a l u m ( I V ) iodide b y t h e reduction of t h e pentaiodide vapour, f o u n d t h a t i f t a n t a l u m m e t a l a t 630° was enclosed w i t h T a l g a t 530-585° ( 1 - 3 a t m pressure T a l g ) t h e sole product was T a l g . N o detailed e x a m i n a t i o n o f its properties appear t o have been made. T a n t a l u m tri-iodide has also been reported as a b y - p r o d u c t o f t h e production o f high p u r i t y t a n t a l u m b y t h e iodide m e t h o d (Rolsten, 1959); d-spacings o f this product have been measured, b u t n o t indexed.

(d) TaJ^^ (Tal^.zs)' This compound, w h i c h was previously u n k n o w n , has recently been m a d e t h e subject of a detailed study. I t is obtained b y t h e reduction o f t a n t a l u m ( V ) iodide vapour under t h e following conditions—(i) b y t h e " t h r e e - t e m p e r a t u r e " m e t h o d : i n this m e t h o d , a sealed quartz t u b e contains t a n t a l u m foil a t a " h i g h - t e m p e r a t u r e "

end (e.g. a t 660°) a n d h q u i d T a l g a t t h e " c o o l " (e.g. 510°) end. T h e T a l g maintains its saturation v a p o u r pressure ('^0-5 a t m ) i n t h e t u b e a n d the vapour reacts w i t h t h e t a n t a l u m t o f o r m Tal4. This last n a m e d

THE HALIDES OF NIOBIUM AND TANTALUM 173

diffuses a w a y from t h e t a n t a l u m m e t a l a n d d i s p r o p o r t i o n a t e s a t t h e i n t e r m e d i a t e t e m p e r a t u r e of t h e middle zone of t h e t u b e (e.g. 528°):

16Tal4(g) = Taeli4(s) + 10Tal5(g)

Taeli4 o b t a i n e d in t h i s w a y forms a grey-black crystalline crust w i t h a metallic l u s t r e ; (ii) b y t h e r e d u c t i o n of T a l g w i t h metallic t a n t a l u m in a simple sealed t u b e ; in t h i s m e t h o d , t a n t a l u m foil is h e a t e d t o 630°

a t one e n d of a sloping q u a r t z t u b e w h i c h also c o n t a i n s a n excess of liquid T a l g a t 575°. After 2 d a y s t h e t u b e is o p e n e d a n d t h e excess of p e n t a i o d i d e r e m o v e d u n d e r v a c u u m . I n a d d i t i o n t o a little u n r e a c t e d t a n t a l u m foil, t h e residue consists of a silvery black m a s s of Ta6li4 crystals ( B a u e r et al., 1965).

Tagli4 is d i a m a g n e t i c , w i t h a susceptibility of Xg = —0-10 χ 10"^

e.m.u. w h i c h is practically c o n s t a n t b e t w e e n 90 a n d 473°K; after allow­

a n c e h a s b e e n m a d e for t h e d i a m a g n e t i s m of t h e ions, t h e r e r e m a i n s a weak, t e m p e r a t u r e - i n d e p e n d e n t p a r a m a g n e t i s m , similar t o t h a t o b ­ served for Nb6Cli4.

T h e X - r a y p o w d e r d i a g r a m c o n t a i n s t o o m a n y lines t o b e i n d e x e d , b u t single-crystal m e t h o d s show t h a t Ta6li4 is o r t h o r h o m b i c , w i t h a = 14-455 A; δ = 12-505 λ; c = 15-000 ( ± 0 - 0 0 5 A), t h e u n i t cell c o n t a i n i n g 4 formula u n i t s . T h e calculated X - r a y d e n s i t y is 7-02 a n d t h e p y c n o m e t r i c d e n s i t y df = 6-85 g cm-^.

Ta6li4 is isotypical w i t h Nb6Cli4, t h e s t r u c t u r e consisting of layers of Ta6li2^'^ u n i t s , e a c h of w h i c h c o n t a i n s a n o c t a h e d r a l core of 6 t a n t a l u m a t o m s , a n d w h i c h a r e b o u n d t o g e t h e r b y t h e iodide anions in a n infinite two-dimensional a r r a y . T h e m e t a l - a t o m o c t a h e d r a a r e n o t q u i t e regular, b u t as in o t h e r halides of t h e s e m e t a l s of formula M6X14, t h e m e t a l - m e t a l a t o m distances s u p p o r t t h e view t h a t t h e m e t a l - a t o m o c t a h e d r a a r e held t o g e t h e r b y m e t a l - m e t a l b o n d s .

(e) Νά^Ιιι{ΝΜι.^^) This c o m p o u n d , in w h i c h t h e formal o x i d a t i o n s t a t e of t h e n i o b i u m is 1 -83, is t h e lowest sub-halide of t h e s e m e t a l s w h i c h h a s b e e n p r e p a r e d , a n d is also t h e first c o m p o u n d of t h e G r o u p V t r a n s i t i o n e l e m e n t s t o b e observed w h i c h contains t h e [MgXg]^ species. I t is formed b y t h e t h e r m a l decomposition of Nbglg u n d e r argon a t 1 a t m p r e s s u r e or b y t h e reaction b e t w e e n n i o b i u m m e t a l a n d n i o b i u m iodides a t 1000°. Single-crystal s t r u c t u r e m e a s u r e m e n t s i n d i c a t e t h a t it c o n t a i n s a basic u n i t consisting of a n o c t a h e d r a l cluster of 6 n i o b i u m a t o m s , w i t h 8 iodine a t o m s located s y m m e t r i c a l l y a b o v e t h e 8 t r i a n g u l a r faces of t h e o c t a h e d r o n ; each of t h e s e u n i t s is joined t o six o t h e r similar u n i t s b y iodine a t o m s e x t e n d i n g radially o u t w a r d s from t h e Nbg o c t a h e d r a . T h e formula can therefore b e w r i t t e n as [Nbglgj^^ll", or [Nbel8]l6/2 ( B a t e -m a n et al, 1966; Schafer et al, 1966).

174 F. FAIRBROTHER

THE HALIDES OF NIOBIUM AND TANTALUM 175

176 F. FAIRBROTHER

THE HALIDES OF NIOBIUM AND TANTALUM 177

178 F. FAIRBROTHER

S h c h u k a r e v , S. Α . , S h e m y a k i n a , T . S. a n d S m i m o v a , E . K . ( 1 9 6 4 ) . Zh. neorg.

Khim. 9, 5 4 7 ; Russ. J. inorg. Chem. 9 , 3 0 4

S h e k a , I . Α . , V o i t o v i c h , B . A . a n d N i s e l ' s o n , L . A . ( 1 9 5 9 ) . Zh. neorg. Khim. 4 , 1 8 0 3 ; Ru^s. J. inorg. Chem. 4 , 8 1 3 .

S h e m y a k i n a , T . S., S m i r n o v a , E . K . a n d S h c h u k a r e v , S. A . ( 1 9 6 2 ) . Vestn.

Leningr. Gos. Univ. Ser. Fiz-i-khim. 1 5 5 .

S i m o n , Α . , v o n S c h n e r i n g , H . G., W o h r l e , H . a n d S c h a f e r , H . ( 1 9 6 5 ) . In S c h a f e r et al. ( 1 9 6 5 b ) .

S k i n n e r , H . A . a n d S u t t o n , L . E . ( 1 9 4 0 ) . Trans. Faraday Soc. 3 6 , 6 8 1 .

S o i s s o n , D . J . , M c L a f f e r t y , J . L . a n d P i e r r e t , J . A . ( 1 9 6 1 ) . Ind. Engng Chem. 5 3 , 8 6 1 .

S p i t z y n , V . I . a n d P r e o b r a s h e n s k i , N . A . ( 1 9 4 0 ) . Zh. ohshch. Khim. 1 0 , 7 8 5 . S t e e l e , B . R . a n d G e l d a r t , D . ( 1 9 5 7 ) . " E x t r a c t i o n a n d R e f i n i n g o f t h e R a r e r

M e t a l s " , S y m p o s i u m , I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y , p . 2 8 7 . S u e , P . ( 1 9 3 9 ) . Bull. Soc. chim. Fr. 6, 8 3 0 .

V a u g h a n , P . Α . , S t u r d i v a n t , J . H . a n d P a u l i n g , L . ( 1 9 5 0 ) . J. Am. chem. Soc. 7 2 , 5 4 7 7 .

V a r g a , L . P . a n d F r e u n d , H . ( 1 9 6 2 ) . J . phys. Chem. 6 6 , 2 1 .

V o i t o v i c h , B . A . a n d B a r a b a n o v a , A . S. ( 1 9 6 1 ) . Zh. neorg. Khim. 6, 1 0 7 3 . W e i n l a n d , R . F . a n d S t o r z , L . ( 1 9 0 6 ) . Ber. dt. chem. Ges. 3 9 , 3 0 5 7 . W e i n l a n d , R . F . a n d S t o r z , L . ( 1 9 0 7 ) . Z. anorg. Chem. 5 4 , 2 2 3 .

W e n t w o r t h , R . A . D . a n d B r u b a k e r , C. H . , J r . ( 1 9 6 2 ) . Inorg. Chem. 1, 9 7 1 . W e n t w o r t h , R . A . D . a n d B r u b a k e r , C. H . , J r . ( 1 9 6 3 ) . Inorg. Chem. 2 , 5 5 1 . W e n t w o r t h , R . A . D . a n d B r u b a k e r , C. H . , J r . ( 1 9 6 4 ) . Inorg. Chem. 3 , 4 7 . W e r n e t , J . ( 1 9 5 2 ) . Z. anorg. allg. Chem. 2 6 7 , 2 1 3 .

W e r n i n g , J . R . , H i g b i e , K . B . , G r a c e , J . T . , S p e e c e , B . F . a n d G i l b e r t , H . L . ( 1 9 5 4 ) . Ind. Engng Chem. 4 6 , 6 4 4 .

W i l l i a m s , M. B . a n d H o a r d , J . L . ( 1 9 4 2 ) . J . Am. chem. Soc. 6 4 , 1 1 3 9 . W i s e m a n , E . L . a n d G r e g o r y , N . W . ( 1 9 4 9 ) . J. Am. chem. Soc. 7 1 , 2 3 3 4 . Y o u n g , R . C. a n d B r u b a k e r , C. H . , J r . ( 1 9 5 2 ) . J . Am. chem. Soc. 7 4 , 4 9 6 7 . Y o u n g , R . C. a n d H a s t i n g s , T . J . ( 1 9 4 2 ) . J . Am. chem. Soc. 6 4 , 1 7 4 0 . Z a l k i n , A . a n d S a n d s , D . E . ( 1 9 5 8 ) . Acta cryst. 1 1 , 6 1 5 .