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Downloaded by EAST CAROLINA UNIV on September 23, 2015 | http://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch011
High- and Low-Valence Metal Cluster Compounds: A Comparison F. A L B E R T COTTON Texas A&M University, Department of Chemistry, College Station, T X 77843
The characteristic differences between metal cluster compounds having the metal atoms in low formal oxidation states and those having them in high formal oxidation states are reviewed critically and analytically. There i s now not only a great number but a l s o a great v a r i e t y of metal atom c l u s t e r compounds. In t h i s essay I should l i k e to d i s c u s s the d i f f e r e n c e s between those that have metal atoms i n a r e l a t i v e l y high mean o x i d a t i o n s t a t e (+2 to +4, and even, i n r a r e cases, a b i t higher) and those with metal atoms i n o x i d a t i o n s t a t e s i n the range -1 to +1. To keep the d i s c u s s i o n w i t h i n reasonable l i m i t s I s h a l l r e s t r i c t i t almost e x c l u s i v e l y to c l u s t e r s c o n s i s t i n g of only two or three metal atoms. I eschew the pedantic a s s e r t i o n that two atoms do not a c l u s t e r make. For convenience and b r e v i t y i n the d i s c u s s i o n the abbreviations HVC and LVC f o r high-valent c l u s t e r and low-valent c l u s t e r , r e s p e c t i v e l y , w i l l be adopted. To i l l u s t r a t e the d i s t i n c t i o n between the two types of c l u s t e r compounds the examples shown i n F i g . 1 may be examined. While the HVC/LVC dichotomy has rather r o u t i n e l y been taken f o r granted i n the past, i t has not been the subject of an explanatory d i s c u s s i o n . There are f i v e p o i n t s of comparison and/or contrast to which I should l i k e to draw a t t e n t i o n ; four are e m p i r i c a l and one t h e o r e t i c a l . Ligand Preferences. The LVC* s are e l e c t r o n r i c h and i n order to e x i s t i n s t a b l e compounds they r e q u i r e l i g a n d s , such as CO, that a r e weak donors and good π acceptors. On the other hand the HVC c l u s t e r s are not a t t r a c t i v e to such l i g a n d s nor do they r e q u i r e them f o r s t a b i l i t y . In t h i s respect, there i s a d i r e c t p a r a l l e l to mononuclear compounds where M° p r e f e r s CO, RNC or s i m i l a r l y π-acidic l i g a n d s while the M (n = 2-4) ions do not g e n e r a l l y form CO complexes. n +
0097-6156/83/0211-0209$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y
HVCs
LVCS
Ο C
CI
CI
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Downloaded by EAST CAROLINA UNIV on September 23, 2015 | http://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch011
I χι l^ci CI
0 D(Re-Re) = 3 . 0 2 Â
CI
D(Re=Re) = 2 . 2 4 Â
V Mo.
foo.
υ
0 H
iOH
I ,2.80A and the Mo-Mo s i n g l e bond i n ( n - C H ) 2 M o 2 ( C 0 ) has the a s t o n i s h i n g length of 3.22Â. Among m u l t i p l e bonds, s i m i l a r trends are found. For ΜΟΞΜΟ t r i p l e bonds we have a l e n g t h of 2.22A f o r the HVC species [ M o ( H P O i ) ] " " but 2.45Â i n the LVC species ( n - C H ) M o (CO) . I t i s appropriate to s t r e s s that metal-metal bonds show f a r more complex behavior than those between main-group atoms and simple bond-length/bond-order r e l a t i o n s h i p s do not e x i s t except i n s p e c i a l cases. Nevertheless, the general trend of longer bonds i n the LVCs as compared to the HVCs i s unmistakable. Any attempt to e x p l a i n t h i s observation must be s p e c u l a t i v e , but i t i s l i k e l y that two i n t e r r e l a t e d f a c t o r s are l a r g e l y responsible. In the HVCs the bonding i s due to overlap of r e l a t i v e l y compact metal di o r b i t a l s f o r which overlap (and hence bond strength) increases markedly w i t h c l o s e approach of the metal atoms. In the LVCs the M-M bonding i s l a r g e l y due to overlap of JS, £ or s£ h y b r i d o r b i t a l s , which are more d i f f u s e and give best overlap at r e l a t i v e l y long d i s t a n c e s . The
