T h e d i v a l e n t s t a t e of m o l y b d e n u m a n d t u n g s t e n is q u i t e different in its halogen c h e m i s t r y t o t h e o t h e r o x i d a t i o n s t a t e s of t h e m e t a l s . T h e halides a n d t h e i r complexes are p o l y n u c l e a r w i t h s t r o n g m e t a l - m e t a l b o n d i n g w i t h i n a n o c t a h e d r a l m e t a l cluster. T h e c h e m i s t r y found here is closely similar t o t h a t r e p o r t e d for t h e polynuclear halides of n i o b i u m a n d t a n t a l u m a n d t e r v a l e n t r h e n i u m . A n excellent review of t h e early w o r k on t h e d i v a l e n t halides is given in Gmelin (1933, 1935).
T h e r e is n o evidence for a difluoride of t h e t w o m e t a l s . T h e reactions of m o l y b d e n u m ( I I ) a n d t u n g s t e n ( I I ) b r o m i d e s w i t h h y d r o g e n fluoride give in each case t h e hexafluorides (Emeléus a n d G u t m a n n , 1949, 1950).
T h e d i v a l e n t halides are p r e p a r e d in general b y either r e d u c t i o n or t h e r m a l decomposition of higher halides. T h e m e t h o d s e m p l o y e d h o w ever v a r y g r e a t l y in yields a n d p u r i t y of t h e p r o d u c t s o b t a i n e d .
I m p u r e samples of m o l y b d e n u m ( I I ) chloride are o b t a i n e d b y t h e t h e r m a l d i s p r o p o r t i o n a t i o n of m o l y b d e n u m ( I I I ) chloride (Couch a n d B r e n n e r , 1959), or b y fusion of t h e p e n t a - a n d trichlorides w i t h m o l y b d e n u m m e t a l (Hellriegel, 1941; Senderoff a n d B r e n n e r , 1954). H o w e v e r , purification is readily achieved b y dissolving t h e c r u d e p r o d u c t in hydrochloric acid, isolating t h e chloro-acid a n d h e a t i n g it t o 200°
in vacuo (Sheldon, 1960b). T u n g s t e n ( I I ) chloride is m o r e difficult t o o b t a i n . R e d u c t i o n of t u n g s t e n hexachloride w i t h a l u m i n i u m p o w d e r a p p e a r s t o be inefficient ( J . Ε . Fergusson, Β . H . R o b i n s o n a n d C. J . Wilkins, u n p u b l i s h e d ) , a n d h y d r o g e n or p h o s p h o r u s gives i m p u r e p r o d u c t s
250 J. Ε. FERGUSSON
(Culbertson a n d Mattingly, 1962); E m e l é u s a n d G u t m a n n , 1950;
N o v i k o v etal., 1961). T h e best m e t h o d of p r e p a r a t i o n a p p e a r s t o be t h e t h e r m a l disproportionation of t h e tetrachloride a t 450-500° (McCarley a n d B r o w n , 1964).
T h e r m a l disproportionation of higher b r o m i d e s of m o l y b d e n u m , or oxidation of m o l y b d e n u m m e t a l w i t h b r o m i n e gives t h e d i v a l e n t b r o m i d e (Gmelin, 1935; D u r a n d et al., 1959). F u s i o n of t h e d i v a l e n t chloride w i t h l i t h i u m b r o m i d e is also a satisfactory p r e p a r a t i v e r o u t e (Sheldon, 1962a). T h e t u n g s t e n ( I I ) b r o m i d e is o b t a i n e d b y r e d u c t i o n of t h e p e n t a b r o m i d e , or in a p u r e s t a t e b y t h e r m a l disproportionation of t h e t e t r a b r o m i d e or u n s t a b l e t r i b r o m i d e (Brown a n d McCarley, 1964;
E m e l é u s a n d G u t m a n n , 1950; McCarley a n d B r o w n , 1962, 1964).
M o l y b d e n u m ( I I ) iodide is readily o b t a i n e d b y t h e r m a l decomposition of t h e tri-iodide (Klanberg a n d Kohlschiitter, 1960; Lewis et al., 1960), or b y reaction of m o l y b d e n u m a n d iodine a t a t e m p e r a t u r e a b o v e which t h e tri-iodide decomposes. F u s i o n of t h e chloride w i t h lithium iodide (Sheldon, 1962a), a n d t h e reaction of either m o l y b d e n u m ( V I ) or (IV) oxide w i t h a l u m i n i u m tri-iodide a t 230° can also be used (Chaigneau, 1957). T h e t u n g s t e n c o m p o u n d is less well defined. R e d u c t i o n of t u n g s t e n ( V I ) chloride w i t h h y d r o g e n iodide v a p o u r or oxidation of t h e m e t a l w i t h iodine a t red h e a t are claimed t o give t h e c o m p o u n d (Gmelin, 1933).
Because of t h e p a r t i c u l a r s t r u c t u r a l t y p e t h o u g h t t o exist for t h e dihalides, it will be convenient t o discuss it briefiy a t this point, in general t e r m s . B y inference from t h e compositions of certain complexes of t h e halides, a n d from spectral a n d s t r u c t u r a l work, it is considered t h a t t h e halides consist of a basic u n i t A^Xg, depicted in F i g . 2 (the letter A will be used w h e n b o t h m o l y b d e n u m a n d t u n g s t e n are referred
cC
F I G . 2. T h e s t r u c t u r e o f t h e M o e C l g c l u s t e r ( n o t a l l o f t h e M o — C I b o n d s a r e s h o w n ) .
HALIDE CHEMISTRY OF CHROMIUM, MOLYBDENUM AND TUNGSTEN 251
TABLE I X . M o l y b d e n u m ( I I ) a n d t u n g s t e n ( I I ) h a l i d e s
C o m p o u n d F o r m a t i o n d a t a C o l o u r M — X
-AH° -A0° B o n d
( k c a l mole~^) ( k c a l mole~^) (e.u.) e n e r g y
( k c a l m o l e - ^ )
M o ( I I ) c h l o r i d e 69, 3 5 31 y e l l o w
M o ( I I ) b r o m i d e 6 2 - 4 , 36^ 5 3 , 2iy 3 1 - 4 , 31^ y e l l o w
-r e d
M o ( I I ) i o d i d e 2 5 (1200° K ) c 13-6 (1200° K ) c 9-5 (1200° K ) c b l a c k 8 9 - 4 C
2 7 17 3 3
W ( I I ) c h l o r i d e 3 6 2 7 3 0 g r e y
W ( I I ) b r o m i d e 2 6 17 31
W ( I I ) i o d i d e 16 6 3 2 b l a c k
T h e t h e r m o d y n a m i c d a t a f o r t h e d i h a l i d e s a r e b a s e d o n a m o n o m e r i c f o r m u l a t i o n . V a l u e s u n d e r l i n e d a r e e s t i m a t e d (cf. B r e w e r et al., 1 9 5 0 ) .
a S h c h u k a r e v et al. ( 1 9 6 0 b ) . c A l l e n a n d Y o s t ( 1 9 5 4 ) . to S h c h u k a r e v et al. ( 1 9 6 2 a ) .
Molybdenum(II) chloride is t h e most soluble of t h e halides in acids and organic solvents. The tungsten dihalides are much less soluble, and this m a y be the reason, at present, for the lack of any extensive chemistry.
to together in a formula). The metal atoms are arranged in an octa
hedron with direct metal-metal bonds, and at the corners of a cube surrounding this octahedron are the eight halogens, each of which bridge three metal atoms. This m a y be w h y no divalent fluorides exist.
This configuration leaves each metal atom free t o bond one more group, four of which are t h e remaining halogens required t o give electrical neutrality. The other t w o molybdenum atoms m a y bridge via halogens from neighbouring cages. Hence the divalent halides can be represented as [ A 6 X 8 ] X 4 . The central nucleus is often depicted as [A^Xg]^"^, which is incorrect because the four external halogens are n o t ionically bound, although they can be readily replaced. Mixed halides of molybdenum(II) are readily prepared [ Μ θ 6 Χ 8 ]Υ4 for all combinations of X and Y (Cl, Br, I). They are obtained b y dissolving t h e halide [ Μ θ 6 Χ 8 ] Χ 4 i n t h e acid H Y and finally removing excess acid b y heating under vacuum (Gmelin, 1935; Sheldon, 1962a). This has not been done t o a n y great extent for tungsten, and in fact the work to be described below largely refers t o molybdenum.
The older literature describes t h e compounds as trimeric on the basis of conductivity data. Pauling (1947) was the first t o predict that the compounds would be hexameric. Table I X summarizes thermodynamic data on t h e halides.
252 J. Ε. FERGUSSON
R e p l a c e m e n t of t h e four e x t e r n a l halogen a t o m s b y anions o t h e r t h a n halogens is possible a n d , for e x a m p l e , h y d r o x i d e , n i t r a t e , s u l p h a t e a n d p e r c h l o r a t e complexes are k n o w n (Lindner, 1927; Gmelin, 1935; D u r a n d et al, 1959; Sheldon, 1962a; A. Brodie, J . E . F e r g u s s o n a n d C. J . Wilkins, u n p u b l i s h e d ) . T h e halides are a t t a c k e d b y m o i s t u r e giving t h e hydrolysis p r o d u c t [ M o X 6] ( O H) 4 a n d further mild hydrolysis gives strongly reducing solutions containing [ Μ θ 6 Χ 8 _ η( Ο Η ) ^ ] ( Ο Η) 4 (Sheldon, 1960a, 1961a, 1962a,b, 1964). Crystals of ( E t 4 N ) 2 [ M o 6 C l 7 0 H ] C l 6 a n d
( E t 4 N ) 2 [ M o 6 C l 4 B r 3 0 H ] B r 6 h a v e been isolated. U n d e r m o r e e x t r e m e con
ditions t h e cage is d i s r u p t e d b y t h e nucleophiles O H " , F " , a n d C N S "
(Sheldon, 1959). T u n g s t e n dihalides a p p e a r t o r e a c t in t h e s a m e w a y (Sidgwick, 1950). M o l y b d e n u m ( I I ) b r o m i d e decomposes u n d e r s t r o n g h e a t i n g (1000°) (Vasil'kova et al, 1964c).
T h e MogXg cluster h a s its o w n chemistry, so m u c h so t h a t it can b e called a pseudo a t o m . T h e dichloride, which h a s been m o s t extensively studied, forms a wide r a n g e of o c t a h e d r a l c o m p o u n d s e x t e r n a l t o t h e cage w i t h m o n o d e n t a t e ligands. T h e halides dissolve in halogen acids t o give [MogXglXe^" ^iid w i t h n e u t r a l ligands complexes [ M o 6 C l 8 ] X 4 Y 2
are formed as n o n electrolytes ( X= : C 1 , B r , a n d I ) . T h e ligand Y ranges from σ-bonding ligands P h a P O t o 7r-bonding P h g P (Lindner, 1927;
Sheldon, 1960a, 1961b, 1962a; E d w a r d s , 1964; F e r g u s s o n et al, 1964, 1967; C o t t o n a n d Curtis, 1965).
T h e p e r c h l o r a t e [ M o 6 C l 8 ] ( C 1 0 4 ) 4 does n o t form such c o m p o u n d s , p r o b a b l y because of t h e g r e a t e r ionic c h a r a c t e r of t h i s c o m p o u n d . Complexes
{ [ M o 6 C l 8 ] L 6 } ( C 1 0 4 ) 4 h a v e been formed (L = d i m e t h y l f o r m a m i d e a n d d i m e t h y l s u l p h o x i d e ; C o t t o n a n d Curtis, 1965). I n a d d i t i o n t h e poly-d e n t a t e s , poly-dipyripoly-dyl, o-phenylene(bispoly-dimethyl)arsine, o-phenanthroline, bis(diphenylphosphino)ethane, a n d t e r p y r i d y l coordinate t o give ionic c o m p o u n d s { [ M o 6 C l 8 ] X 2 ( b i d e n t a t e 2 ) } X 2 a n d {[MoeClglXstridentatejX (Fergusson et al,, 1964, 1967; Clark et al, 1966). Similar complexes w i t h
[ W 6 C l 8 ] C l 4 h a v e been p r e p a r e d w i t h o-phenanthroline a n d o-phenyl-(bisdimethyl)arsine a n d halogen ligands (Lindner a n d K o h l e r , 1924;
Clark et al, 1966). I n all cases of complex formation, t h e n u m b e r of d o n o r a t o m s covalently b o u n d e x t e r n a l t o t h e cage is i n v a r i a b l y six.
T h e m a n n e r in which t h e cage can cope w i t h t h e p o l y d e n t a t e s is n o t y e t fully u n d e r s t o o d , b u t m a y involve r e m o v a l of certain of t h e bridging halogens (Fergusson et al, 1964, 1967). This h a s b e e n found possible in t h e hydrolysis of t h e halides irrespective of t h e a p p a r e n t inertness t o ex
change of t h e bridging halogens (Sheldon, 1960b). A l t e r n a t i v e l y , d e -localized molecular orbitals of t h e cage m a y accept u p t o six electron pairs n o t necessarily placed s y m m e t r i c a l l y a r o u n d t h e cage (Fergusson et al, 1964, 1967).
HALIDE CHEMISTRY OF CHROMIUM, MOLYBDENUM AND TUNGSTEN 253
T h e h a h d e s a n d their complexes a r e all d i a m a g n e t i c ( K l e m m a n d Steinberg, 1936; Sheldon, 1960a; F e r g u s s o n et al., 1964, 1967). L o w -frequency infrared studies h a v e shown t h a t t w o b a n d s in t h e 250-360 c m - i region m a y b e associated w i t h bridging halogens in t h e complexes
[ A g C l g l X e ' - a n d {[A6Cl8]Cl2(bidentate)2}Cl2 (Clark et al., 1966).
The structures of t h e compounds H2[Mo6Cl8]Cl6, [Mo6Cl8]Cl4.2H20, [Mo6Cl8](OH)4.2H20 (Brosset, 1945, 1946, 1950) a n d (NH4)2[Mo6Cl8]
C l g H g O (Vaughan, 1950), h a v e been elucidated a n d are t h e basis of t h e description given earlier. Typical b o n d lengths a r e : Mo—Mo 2-64 Â, M o — C l b r i d g e 2-54 Â, a n d M o — C l t e r m i n a i 2-45 Â. T h e m e t a l - m e t a l b o n d is close t o t h a t e x p e c t e d for a single b o n d a n d r a t h e r surprisingly t h e t e r m i n a l m e t a l halogen b o n d s a r e shorter t h a n t h e bridging m e t a l -halogen b o n d s . This m u s t , if correct, b e a consequence of t h e p a c k i n g r e q u i r e m e n t s w i t h i n t h e cage. I t a p p e a r s necessary t o h a v e further s t r u c t u r a l information, employing a complete s t r u c t u r a l analysis, o n k e y c o m p o u n d s . T h e dihalides themselves a r e a m o r p h o u s t o X - r a y s . I t is reasonable t o assume t h a t t h e dihalides a n d their complexes h a v e t h e same s t r u c t u r e as t h e c o m p o u n d s a b o v e . T h e c o m p o u n d s [Μθ6Χ8](ΟΗ)4(Η2θ)2.12Η2θ ( X = Cl a n d Br) are isomorphous (Sheldon,
1962a), as also a r e t h e complexes H2[A6Cl8]X6.8H20 ( X = CI, B r ) , which d e m o n s t r a t e s t h a t t h e cage exists in t h e t u n g s t e n c o m p o u n d s (Clark et al., 1966). I n a d d i t i o n t h e electronic a b s o r p t i o n spectra of t h e halides a n d complexes a r e v e r y closely similar (Table X , p . 254). Shifts occur, as t h e bridging halogens a r e changed, in t h e expected w a y (Sheldon, 1960a; Fergusson et al., 1964, 1967). T h e b o n d i n g in t h e clusters is of interest i n view of t h e c u r r e n t g r o w t h i n t h e s t u d y of m e t a l -m e t a l b o n d e d s t r u c t u r e s . I t h a s been suggested t h a t t h e oxidation s t a t e of t h e m e t a l is (VI) r a t h e r t h a n ( I I ) (Sheldon, 1960a). However, i t is p r o b a b l y meaningless t o consider oxidation s t a t e s in such c o m p o u n d s . T h e t w o b o n d i n g descriptions outlined a r e t h e valence b o n d a n d mole
cular orbital. T h e former m e t h o d uses s q u a r e a n t i p r i s m h y b r i d s d^sp^ of t h e m o l y b d e n u m a t o m (Gillespie, 1961). Such a b o n d i n g scheme does i m p l y t h e u s e of b e n t b o n d s , a n d in fact it is difficult t o see h o w t h e bridging halogens c a n b o n d in a n y o t h e r w a y . A molecular orbital description based o n a p a r t i a l h y b r i d scheme a p p e a r s t o b e satisfactory for t h e complete cage (Grossman et al., 1963). R e c e n t l y , a n o c t a h e d r a l molecular orbital description of t h e m e t a l - m e t a l b o n d i n g h a s been used w i t h success. T h e four bridging halogens, a r o u n d each m e t a l , are a s s u m e d t o use u p t h e d^y, s, p^, a n d ^ ^ orbitals of each m e t a l . This leaves a t o t a l set of b o n d i n g orbitals ^ ι^ ( 4 ' ) T^^{dç^^dy^) T^gid^^y^) Eg{d^^.y^ a n d
T2u{da'!^-y-) which a c c o m m o d a t e t h e 24 d electrons of t h e six m e t a l a t o m s (Cotton a n d H a a s , 1964).
254 J. Ε. FERGUSSON
C o m p o u n d A b s o r p t i o n b a n d s (ιημ)^
A Β C
[ M o e C l g l C l ^ . S H g O 3 1 7 3 4 2 3 5 9
[MOfiClglCl^ 316 346 357
[ M o e C l s J l ^ 3 1 8 4 5 0 4 7 0
[ M o e B r g J B r ^ 341 3 7 3 3 9 0
[ M o e I « ] I , 3 6 9 4 5 8 4 7 2
[ M o e C l e l C l ^ i P h g P ) ^ 3 1 9 3 4 9 356
{[MoeCl8]Cl2(dipy)2}Cl2 3 1 8 3 4 3 357 374t>
^ Diffuse reflectance spectra.
^ A n additional b a n d occurs for t h e p o l y d e n t a t e ligands.
R e c e n t l y , t h e complexes KgMogClig, ( Ν Η 4 ) 7 Μ θ 3 θ 1 ΐ 3 . Η 2 θ , a n d
C s 6 M o 4 X i 6 ( X = CI, B r , I) were p r e p a r e d from m o l y b d e n u m ( I I ) a c e t a t e in acid solution. T h e first t w o c o m p o u n d s contain m o l y b d e n u m in a formal oxidation s t a t e of I I . On t h e basis of m a g n e t i s m , electronic a b s o r p t i o n spectra, a n d composition, t h e c o m p o u n d s are said t o b e b a s e d on clusters
M03CI13
a n dM04CI16
b y a n a l o g y w i t h t h e r h e n i u m clusters (Anderson a n d Sheldon, 1965). T h e anions are u n e x p e c t e d in view of t h e v e r y high charge associated w i t h t h e m . F u r t h e r studies on these c o m p o u n d s should p r o v e revealing, especially as it is n o w k n o w n t h a t m o l y b d e n u m ( I I ) a c e t a t e is dimeric w i t h a direct Mo—^Mo b o n d . C. Molybdenum and Tungsten(III) (d^)(i) Halides
W i t h p e r h a p s t h e exception of t h e b r o m i d e , t h e trihalides of m o l y b d e n u m a n d t u n g s t e n are restricted t o those of t h e lighter m e t a l .
M o l y b d e n u m ( I I I ) fluoride h a s been o b t a i n e d b y t r e a t i n g either t h e trichloride or t r i b r o m i d e w i t h h y d r o g e n fiuoride (Emeléus a n d G u t m a n n , 1949). T h e reaction, w h i c h is h i n d e r e d b y t h e formation of a coating of t h e fiuoride over t h e s t a r t i n g m a t e r i a l , could n o t be confirmed. R e d u c t i o n of m o l y b d e n u m ( V ) fluoride w i t h either m o l y b d e n u m m e t a l or a n t i m o n y trifluoride gives t h e trifluoride which can b e r e m o v e d from t h e s t a r t i n g m a t e r i a l b y distillation (La Valle et al., 1960). T h e t r e a t m e n t of m o l y b d e n u m m e t a l w i t h h y d r o g e n fluoride is n o t a v e r y successful m e t h o d of p r e p a r a t i o n (Muetterties a n d Castle, 1961).
T h e trichloride is best o b t a i n e d b y reducing m o l y b d e n u m p e n t a chloride w i t h m o l y b d e n u m m e t a l (Hellriegel, 1941; Campbell, 1959), h y d r o g e n (Couch a n d B r e n n e r , 1959; Culbertson a n d M a t t i n g l y , 1962), or w i t h a n h y d r o c a r b o n (Kovacic a n d L a n g e , 1963; SenderoflF a n d L a b r i e , 1955). T h e r e is d o u b t in t h e l a t t e r case a b o u t h o w far t h e
TABLE X . E l e c t r o n i c a b s o r p t i o n b a n d s o f m o l y b d e n u m ( I I ) h a l i d e s a n d c o m p l e x e s
HALIDE CHEMISTRY OF CHROMIUM, MOLYBDENUM A N D TUNGSTEN 255
r e d u c t i o n proceeds (Larson a n d Moore, 1964). O x i d a t i o n of t h e m e t a l using m e r c u r o u s chloride h a s been used (Bettinger, 1961). Electrolytic r e d u c t i o n of m o l y b d e n u m (VI) i n hydrochloric acid gives m o l y b d e n u m trichloride i n solution, a useful source for further reactions a s t h e solid halide is n o t soluble in acids.
M o l y b d e n u m ( I I I ) b r o m i d e is o b t a i n e d from t h e m e t a l w h e n t r e a t e d w i t h b r o m i n e (350-400'^) (Rosenheim et al, 1931; D u r a n d et al, 1959;
F e r n a n d e z - M a s a g u e r a n d D u r a n , 1959), or m e r c u r o u s b r o m i d e (Bettinger, 1961). R e d u c t i o n of t h e t e t r a b r o m i d e also gives t h e green c o m p o u n d .
T h e tri-iodide h a s been o b t a i n e d b y reacting t h e elements i n a sealed t u b e a t 300° (Lewis et al, 1960). I f t h e r e a c t i o n t e m p e r a t u r e is t o o high it a p p e a r s t h a t t h e di-iodide is formed. Gaseous h y d r o g e n iodide is said t o reduce t h e pentachloride t o m o l y b d e n u m tri-iodide in carbon disulphide, whereas liquid H I h a s n o effect ( K l a n b e r g a n d Kohlschiitter, 1960).
All a t t e m p t s t o p r e p a r e t u n g s t e n trifluoride were unsuccessful (Emeléus a n d G u t m a n n , 1950; M u e t t e r t i e s a n d Castle, 1961). I t does a p p e a r , however, t h a t if t u n g s t e n ( I I ) b r o m i d e is left i n c o n t a c t w i t h liquid b r o m i n e for a long period a t 50° a b l a c k c o m p o u n d WBrg is formed which quickly decomposes t o t h e d i v a l e n t b r o m i d e a b o v e 80°
(Brown a n d McCarley, 1964; McCarley a n d B r o w n , 1962).
T h e trihalides (Table X I ) of m o l y b d e n u m a r e insoluble i n w a t e r . I f h e a t e d i n air t h e fluoride a n d chloride oxidize t o t h e m o l y b d e n u m ( V I ) oxide, a n d i n t h e case of t h e chloride, m o l y b d e n y l chloride also forms ( S h c h u k a r e v et al, 1960b). T h e trifluoride is stable t o h e a t a n d only in a d y n a m i c v a c u u m a t 600° does decomposition occur (Emeléus a n d G u t m a n n , 1949; L a Valle et al, 1960), w h e r e a s t h e trichloride decomposes before melting t o give t h e t e t r a - , di-, a n d pentachlorides, a n d m o l y b d e n u m m e t a l . T h e colour of m o l y b d e n u m ( I I I ) fluoride a p p e a r s t o b e t e m p e r a t u r e d e p e n d e n t (La Valle et al, 1960).
8 0 0 ° . 1 , ι ι ; Γ „ Τ : ι 9 0 0 °
t a n M o F o . black M o F , . r e d M o F o
TABLE X I . M o l y b d e n u m ( I I I ) h a l i d e s
C o m F o r m a t i o n d a t a S° M e l t i n g C o l o u r
p o u n d - A H ° - A S ° (e.u.) p o i n t
( k c a l ( k c a l (e.u.)
m o l e- i ) mole~^) (°C)
M0CI3 94, 6 5 a 5 0 4 3 , 4 9 » 32-6^ 1 0 2 7 b r o w n - r e d
M o B r g 64, 4 1 , 5 i c , d 3 6 , 3 4 c 51 3 9- 3 C 9 7 7 g r e e n
M0I3 37, 15c 2 2 , 15c 52 4 9- 8 C 9 2 7 b l a c k
V a l u e s u n d e r l i n e d a r e e s t i m a t e d (cf. B r e w e r et al., 1 9 5 0 ) . a S h c h u k a r e v et al. ( 1 9 6 0 a ) . c L a t i m e r ( 1 9 5 2 ) . i> D r o z i n ( 1 9 6 1 ) . d S h u k u r o v et al. ( 1 9 6 1 ) .
256 J. Ε. FERGUSSON
T h e changes m a y well be associated w i t h s t r u c t u r a l modifications.
W i t h liquid b r o m i n e t h e t r i b r o m i d e is oxidized t o m o l y b d e n u m ( I V ) b r o m i d e (Carnell a n d McCarley, 1962), a n d w i t h alkylcyanides com
plexes MoBrg.SRCN are formed which are useful s t a r t i n g materials for further c o m p o u n d formation (Allen ef al., 1965).
L i q u i d a m m o n i a brings a b o u t ammonolysis of one Mo—CI b o n d in
M0CI3,
a n d t w o Mo—^Br b o n d s in t h e b r o m i d e , while reactions of t h e t r i b r o m i d e w i t h m e t h y l - a n d d i m e t h y l a m i n e solvolyse t w o a n d one M o — B r b o n d s respectively ( E d w a r d s a n d Fowles, 1962; E d w a r d s , 1964).T h e r e is disagreement on t h e s t r u c t u r a l t y p e for m o l y b d e n u m ( I I I ) fiuoride. I t h a s b e e n described as cubic (ReOg t y p e ) (Emeléus a n d G u t m a n n , 1949; G u t m a n n a n d J a c k , 1951) a n d r h o m b o h e d r a l (VF3 t y p e ) (LaValle et al., 1960), a n d t h e r e is evidence t h a t t h e r h o m b o h e d r a l sample absorbs m o i s t u r e a n d becomes cubic. I t is clear t h a t t h e differ
ences s t e m from either relative p u r i t y or different crystal modifications.
T h e l a t t e r possibility is a well k n o w n feature of o t h e r halides. T h e observations on t h e effect of h e a t on t h e colour of t h e flouride m a y b e r e l e v a n t here. T h e X - r a y p o w d e r p h o t o g r a p h of t h e sample of t h e ReOg t y p e d e m o n s t r a t e s a n o c t a h e d r a l stereochemistry w i t h a M o — F b o n d length of 1·95 Â ( G u t m a n n a n d J a c k , 1951).
T h e trifiuoride of m o l y b d e n u m h a s a low m a g n e t i c m o m e n t (0-53 B) (P. W . S m i t h , J . Lewis a n d R . S. N y h o l m , u n p u b l i s h e d ) . T h e trichloride a p p e a r s t o b e antiferromagnetic a n d h a s a low m a g n e t i c m o m e n t re
p o r t e d as 0-67 Β (293°K), a n d 1-00 Β (300°K), w i t h a Weiss c o n s t a n t of 100. T h e m o m e n t of t h e b r o m i d e is r e p o r t e d as 1-24 Β ( K l e m m a n d Steinberg, 1936; Colton a n d M a r t i n , 1965). T h e s t r u c t u r e of t h e chloride h a s been r e p o r t e d as a d i m e r w i t h each m o l y b d e n u m in o c t a h e d r a l co
ordination. T h e Mo—^Mo d i s t a n c e in t h e dimer is 2-77 Â a n d b e t w e e n dimers is 3-70 Â. T h e Mo—CI b o n d distances are t w o of 2-40, t w o of 2-45, a n d t w o of 2-55 A (Schnering a n d Woehrle, 1963; Schafer a n d Schnering, 1964). T h e short Mo—^Mo distance would explain t h e low m a g n e t i c m o m e n t . R e c e n t l y , t h e c o m p o u n d h a s been predicted t o b e trimeric
M03CI9
on t h e basis of m a g n e t i c a n d spectral d a t a a n d similarity of its p o w d e r p h o t o g r a p h t o t h a t of r h e n i u m ( I I I ) chloride (Colton a n d M a r t i n , 1965). T h e s t r u c t u r a l d a t a , however, do n o t a p p e a r t o s u p p o r t this. Again, different p o l y m o r p h s m a y b e t h e reason. T h e b r o m i d e cont a i n s t h e m o l y b d e n u m in o c t a h e d r a l coordination (Mo—Br = 2-57 Â), a n d a t t i m e s a distance of 3-03 Â occurs b e t w e e n adjacent m o l y b d e n u m a t o m s (Babel a n d Rtidorff, 1964).
T h e m o l y b d e n u m trichloride a n d t r i b r o m i d e h a v e similar electronic a b s o r p t i o n spectra a n d p a r t s of t h e b a n d s t r u c t u r e are similar t o
HALIDE CHEMISTRY OF CHROMIUM, MOLYBDENUM AND TUNGSTEN 257
MogClg^" (Clark, 1964). This m a y be a feature of t h e dimeric n a t u r e of t h e halides. T h e b r o m i d e a n d iodide a r e said t o b e isostructural (Lewis et al, 1960).
(ii) Halide complexes
V e r y little recent w o r k h a s b e e n carried o u t on t h e complex fluoro-anions of m o l y b d e n u m a n d t u n g s t e n ( I I I ) . T h e complexes MM0F4.H2O (M = N a , NH4, K ) , MWF4.H2O (M - NH4, K ) , a n d M3M02F9 (M
= NH4, K ) were r e p o r t e d in early w o r k (Gmelin, 1933, 1935). T h e complex KgMoFg is a b r o w n solid o b t a i n e d b y fusing KgMoCle w i t h p o t a s s i u m h y d r o g e n fluoride. I t is cubic a n d h a s a m a g n e t i c m o m e n t of 3-2 Β (Peacock, 1960).
T h e r e d chloro complexes MaiMoClg.HaO] a n d M3[MoCl6].M = Li, N a , K , R b , Cs, NH4 (Gmelin, 1935; R a m a s e s h a n a n d S u r y a n , 1951;
L o h m a n n a n d Y o u n g , 1953; I r v i n g a n d Steele, 1957; Senderoif a n d B r e n n e r , 1954; Senderoff a n d L a b r i e , 1955), are o b t a i n e d b y t h e electro
lytic r e d u c t i o n of m o l y b d e n u m ( V I ) oxide in hydrochloric acid or r e d u c t i o n w i t h p o t a s s i u m a m a l g a m . I n t h e former m e t h o d t h e acidity d e t e r m i n e s t h e a c t u a l p r o d u c t . T h e r e p o r t e d h e p t a - a n i o n [MoCl,]^- is p r o b a b l y a double salt. T h e c o m p o u n d s (NH4)3MoCl6.NH4Cl.H20 a n d K3MoCl6.4KCl.6H2O h a v e also b e e n p r e p a r e d . I n t h e s e c o m p o u n d s a coordination n u m b e r g r e a t e r t h a n six is n o t confirmed b y t h e spectral a n d m a g n e t i c d a t a (Irving a n d Steele, 1957). T h e complexes are stable in d r y air a n d hydrochloric acid; otherwise t h e y h y d r o l y s e readily, especially t h e l i t h i u m a n d s o d i u m salts. T h e complexes a p p e a r t o be s t r o n g reducing a g e n t s a n d liquid b r o m i n e a t 45° converts t h e p o t a s s i u m salt t o KaMoClg (Young a n d L a u d i s e , 1956). O x i d a t i o n also occurs in sulphuric acid (Bailin a n d J o n a s s e n , 1963). T h e w a t e r in t h e p e n t a -chloro complexes is replaceable w i t h a m m o n i a .
All t h e recorded m a g n e t i c m o m e n t s a n d electronic a b s o r p t i o n s p e c t r a of t h e complexes are in accord w i t h a d^ spin-free configuration in a n o c t a h e d r a l field ( K l e m m a n d Steinberg, 1936; Griffiths et al, 1953;
E p s t e i n a n d Elliott, 1954; H a r t m a n n a n d S c h m i d t , 1957; I r v i n g a n d Steele, 1957; Figgis et al, 1961; Mitchell a n d Williams, 1962; R u n c i m a n a n d Schroeder, 1962). L i g a n d field p a r a m e t e r s , Dq = 1920 cm-^ a n d Β — 432 cm~^, h a v e b e e n d e d u c e d ( R u n c i m a n a n d Schroeder, 1962).
T h e m a g n e t i c m o m e n t s are on t h e a v e r a g e a b o u t 3 % low, r a t h e r t h a n t h e e x p e c t e d 5-6%, i n d i c a t i n g a r e d u c t i o n in t h e spin-orbit coupling d u e t o possible orbital delocalization (Figgis et al, 1961). K2M0CI5 e x h i b i t s a s h a r p line in t h e e.s.r. s p e c t r u m , g = 1-76 ( R a m a s e s h a n a n d S u r y a n , 1951). T h e b i s a m m o n i u m a n d b i s p o t a s s i u m salts a p p e a r re
l a t e d in s t r u c t u r e t o t h e r h o m b i c i r o n ( I I ) complex (Carabbi, 1928).
258 J. Ε. FERGUSSON
T h e dinuclear species MogClg^- a n d Mo^Br^^- can b e o b t a i n e d b y t r e a t i n g t h e t e r v a l e n t a n i o n MoXg^- w i t h a m m o n i a . T h e cations used t o isolate t h e anions are K , R b , Cs, a n d Rg-a-NHi+a, ( E d w a r d s et al.,
1962a; P . W . S m i t h , J . Lewis a n d R . S. N y h o l m , u n p u b l i s h e d ; G. J . K e l l y a n d P . W . S m i t h , u n p u b h s h e d ) . T h e complexes M2(H30)Mo2Cl9 h a v e also b e e n r e p o r t e d (M = E t 4 N , Ph4As) (P. W . S m i t h , J . Lewis a n d R . S. N y h o l m , u n p u b l i s h e d ) . T h e complexes
M3M02CI9
are i s o s t r u c t u r a l w i t h t h e corresponding t u n g s t e n c o m p o u n d s (see below) a n d also h a v e low m a g n e t i c m o m e n t s . T h e a b s o r p t i o n s p e c t r a a p p e a r related t o t h e trihalides (P. W . S m i t h , J . Lewis a n d R . S. N y h o l m , u n p u b l i s h e d ; T . M. D u n n a n d P . W . S m i t h , u n p u b h s h e d ) .T h e chloro c o m p o u n d s of t u n g s t e n ( I I I ) are e n t i r e l y confined t o t h e dinuclear complexes
M3W3CI9
(M =NH4,
K , R b , Cs, Tl, Co(NH3)63+, C r ( N H 3) 6 3 + , Ag(NH3)2+, p y H , a n d P h N H 3 ) . T h e c o m p o u n d s are isol a t e d from a n acid solution of t u n g s t e n ( I I I ) o b t a i n e d b y electrolytic r e d u c t i o n of t u n g s t e n ( V I ) (Gmelin, 1933; J o n a s s e n a n d Cantor, 1956;
J o n a s s e n et al., 1957; L a u d i s e a n d Y o u n g , 1960). T h e m o n o v a l e n t cations give green-yellow solids which can b e recrystallized from a q u e o u s hydrochloric acid. T h e c o m p o u n d s h y d r o l y s e slowly a n d oxidize in air.
T h e y a p p e a r t o r e d u c e h o t w a t e r t o h y d r o g e n (Young, 1932).
T h e crystal s t r u c t u r e of
K3W2CI9
h a s been s t u d i e d a n d t h e anion is depicted in F i g . 1, p . 241 (Brosset, 1935; P a u l i n g , 1947; W a t s o n a n d Waser, 1958). T h e t u n g s t e n - t u n g s t e n internuclear distance of 2-41 Â is v e r y short, a n d h a s t h e effect of distorting t h e o c t a h e d r a l a r r a n g e m e n t a r o u n d t h e m e t a l , as s h o w n b y t h e W—CI b o n d l e n g t h s : W—Clterminai 2-40 Â, W—Clbridging 2-48 Â. T h e salts of NH4, K , R b , Cs, a n d Tl are all i s o m o r p h o u s . T h e s t r o n g m e t a l - m e t a l i n t e r a c t i o n is also i n d i c a t e d in t h e d i a m a g n e t i s m or low p a r a m a g n e t i s m observed for t h e salts (0-47 B) ( K l e m m a n d Steinberg, 1936; J o n a s s e n a n d Cantor, 1956; W a t s o n a n d W a s e r , 1958). D e s p i t e t h e i n e q u a l i t y of t h e chlorine a t o m s t h e y are all equally labile ( H a w k i n s a n d G a r n e r , 1958). This m a y reflect t h e d o m i n a t i n g influence of t h e s t r o n g m e t a l - m e t a l b o n d . T h e infrared s p e c t r u m of t h e anion h a s a b a n d a t 16-4 /x a t t r i b u t a b l e t o t h i s b o n d a n d a b a n d a t 23-8 μ d u e t o t h e bridging halogens (Jonassen a n d Cantor, 1956).T h e electronic a b s o r p t i o n b a n d s a t 462 a n d 625 m μ (Laudise a n d Y o u n g , 1955) m a y also be associated w i t h t h e m e t a l b o n d i n g (cf.
m o l y b d e n u m ( I I ) p . 253), a n d r h e n i u m ( I I I ) . T h e t w o d i a m a g n e t i c com
p o u n d s W2Cl6(py)3 a n d W2Cl6(PhNH2)3 are p r e s u m a b l y b a s e d on t h e dinuclear s t r u c t u r e (Jonassen a n d Cantor, 1956; J o n a s s e n et al., 1956).
T h e c o m p o u n d K5W3CI14 is claimed t o h a v e b e e n isolated from t h e s a m e solution from which WgClg^- is o b t a i n e d (Laudise a n d Y o u n g , 1955). H o w e v e r , t h e r e is good reason from X- r a y a n d spectral d a t a t o
HALIDE CHEMISTRY OF CHROMIUM, MOLYBDENUM AND TUNGSTEN 2 5 9
believe t h a t t h e c o m p o u n d is a m i x t u r e of K3W2CI9 a n d K2W(OH)Cl5 (Konig, 1 9 6 3 ) . T h e a n i o n s W3Cli7^- a n d Mo3Fi4^- m a y be in t h e s a m e category (Cooper a n d W a r d l a w , 1932; K n o x , 1 9 6 0 b ) .
B r o m o complexes of m o l y b d e n u m of t h e t y p e MoBrg.HgO^" a n d MoBr(5^~ h a v e been p r e p a r e d in t h e usual w a y (Gmelin, 1935). H o w e v e r , v e r y little h a s been d o n e on t h e c o m p o u n d s e x c e p t t h e i r p r e p a r a t i o n . E v i d e n c e from s p e c t r a is in accord w i t h a spin-free configuration in a n o c t a h e d r a l e n v i r o n m e n t ( H a r t m a n n a n d S c h m i d t , 1957). T h e complex CS2M0I5H2O h a s also been isolated ( P . W . S m i t h , J . Lewis a n d R . S.
N y h o l m , u n p u b l i s h e d ) . (iii) Oxyhalides
Oxyhalides of t e r v a l e n t m o l y b d e n u m h a v e b e e n p r e p a r e d b y W a r d l a w a n d W o r m e l l ( 1 9 2 4 ) , W a r d l a w a n d H a r d i n g (1926), W a r d l a w a n d W o r m e l l ( 1 9 2 7 a, b ) , B u c k n a l l etal, ( 1 9 2 7 ) , b u t h a v e n o t been s t u d i e d since T h e c o m p o u n d s M0OCI.4H2O (buff a n d green forms), M o O B r. 4 H 2 0 , K[ M o O C l 2. 3 H 2 0 ] , a n d N H 4 [ M o ( O H ) C l 3. 3 H 2 0 ] are r e p o r t e d t o be pre
p a r e d b y electrolytic r e d u c t i o n of m o l y b d e n u m ( V I ) . T h e oxyfluoride, M o O F . 3- 5 H 2 0 , is p r e p a r e d from t h e chloride b y fusing w i t h a m m o n i u m fluoride.
D. Molybdenum and Tungsten(IV) {d')