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Higher Nuclearity Carbonyl Clusters BRIAN T. H E A T O N University of Kent, Chemical Laboratory, Canterbury CT2 7NH England
Professor Cotton's studies have considerably clarified our understanding of di- and tri-nuclear metal-metal bonded compounds. For higher nuclearity clusters, rationalisation of structures, bonding, reactivity etc., must be much more tenuous because of the increased number of variables (metal-metal, metal-ligand, steric effects) now present. However, I would briefly like to present a few trends which seem to be emerging in this area. Substitution Sites in Tetra- and Penta-Nuclear Clusters Apart from nitrile-substituted derivatives of [Ir (CO) ] (1, 2) which retain the non-carbonyl-bridged structure of [Ir (CO) ], (3) all other derivatives of [Ir (CO) ], both in the solid state and in solution, are based on the C -structure of [M(CO) ][(M = Co, (4-7) Rh (4, 8)](Fig. 1). This structure, with three edge-bridging carbonyls, is better able to dissipate the increased nuclear charge induced on carbonyl substitution. Three possible isomers could result by carbonyl replacement at the apical, radial or axial site but the better back-bonding ability favours carbonyl replacement on the basal metal atom and the substituent is found exclusively in the axial site, eg. [Ir (CO) X] (X = Br, (9) COMe, (10) [Ir(CO) ] (11)), [Ir (CO) L] (L = PR (12)). 4
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Further occugancy of a x i a l s i t e s occurs with small l i g a n d s , eg. [ l r ^ ( C 0 ) ^ Q H ] ~", (13) but minimisation of s t e r i c e f f e c t s becomes important f o r l a r g e r l i g a n d s and the s u b s t i t u t i o n p a t t e r n shown i n F i g . 2 i s p r e f e r r e d . (12, 14) The reason f o r p r e f e r e n t i a l a x i a l s i t e occupancy by monodentate l i g a n d s i s probably r e l a t e d to r e s u l t s of recent CNDO c a l c u l a t i o n s on [ C o ^ i C O ) ^ ] * (15) which show that the a x i a l carbonyls are l e a s t i n v o l v e d i n back-bonding. E x t r a p o l a t i o n to Rh^- and I r ^ - d e r i v a t i v e s seems reasonable and f i n d s some support from n.m.r. measurements, which show that the ^ C n.m.r. chemical s h i f t of the a x i a l carbonyl i s always a t higher f i e l d than the r a d i a l carbonyl i n both rhodium (16) and i r i d i u m d e r i v a t i v e s . (12) 2
0097-6156/83/0211-0227$06.00/0 © 1983 American Chemical Society Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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INORGANIC C H E M I S T R Y :
Figure 2. Isomers formed on ligand (L = PR , P(OPh) ) substitution in [M (CO) ], (M = Rh, Ir). Key: ·, terminal CO; and M, bridging CO. S
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Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Related s u b s t i t u t i o n patterns are observed i n t e t r a n u c l e a r c o b a l t and rhodium c l u s t e r s . Thus, the small l i g a n d , P(OMeK(L), occupies a x i a l s i t e s i n [Co,(CO) L ] (x = 1,2) (17) whereas s t e r i c e f f e c t s become important witS ?(OPhK and the isomers shown i n F i g . 2 are obtained with tetrarhodium d e r i v a t i v e s . U8> υ ) . However, there i s some d e v i a t i o n from these s u b s t i t u t i o n p a t t e r n s , e s p e c i a l l y with cobalt c l u s t e r s . Thus, i o d i d e i n [ C o ^ ( C O ) ^ l ] ~ i s predominantly i n an a p i c a l s i t e , although there i s some s u b s t i t u t i o n (2%) on the b a s a l c o b a l t , (20) and P(CH CH:CH )Me2(L) i n [ C o ( C 0 ) L ] occupies one a x i a l and a trans-apical s i t e . The reasons f o r these d i s c r e p a n c i e s are not c l e a r but are probably a s s o c i a t e d with cobalt being both smaller and l e s s e l e c t r o n e g a t i v e than rhodium or i r i d i u m . _ For the s u b s t i t u t e d penta-nuclear c l u s t e r , [Rh
