Carbonyls, metal Bridging

While essentially all the metal carbonyl complexes for group 4B contain terminal CO ligands, only recently have some bonafide doubly bridging carbonyl complexes been reported. However, these complexes are hetero-nuclear, since the carbonyl ligand bridges a zirconium atom with the metal center of a late transition metal. 

The cluster anion, [Os3Ir(CO)i3], was prepared in 50% yield by reaction of Os3(CO)i2 with [Ir(CO)4]. The single-crystal X-ray structure analysis shows it to consist of a tetrahedral metal core with one of the 13 carbonyl ligands bridging. The catalytic activity for carbonylation of... 

In the case of [Fe3(CO)n]2-, a triply bridging carbonyl group is asymmetrically positioned (as deduced by the bond distances) above the Fe3 triangle, which is not found for [Ru3(CO)i i]2-,18 and [Os3(CO)i i]2-.15 In all the three compounds there is a carbonyl group bridging two metal atoms. 

Four of the eight triangular faces in octahedral Rh6(CO)16 (Rhftfri3-CO)4(CO),. J (Fig. 15.10) contain carbonyl groups bridging three metal atoms. The corresponding cobalt compound is thought to have a similar structure.17... 

Carbyne ligands may bridge two (p) or three (p3) metal centres, providing a total of 3VE to the overall electron count. Earlier synthetic routes to such complexes involved the reactions of carbonyl metallates with 1,1,1-trihaloalkanes, or the cleavage of alkyne ligands coordinated to clusters (Figure 5.46). 

For mononuclear complexes, reactions at coordinated ligands is a topic of major study, and numerous reviews and even books have been written on this topic. At the time of writing this review, the authors could find no major review articles in this area for dinuclear compoimds. The best examples are probably those on the reactions of coordinated molybdenum sulfides discussed earlier. A related observation is the reaction of metal carbonyls with Lewis acids. It has been observed by Shriver and coworkers that carbonyl ligands bridging metal-metal bond atoms show increased tendency to coordinate Lewis acids (equation 96). 

Carbonylation metal salt reduction by CO at high pressure with M-CO bond formation Polyhydride complex of the type L MHj (x > 3) Semibridging CO unsymmetrically bridged CO... 

When complexes with two metals bridged by an h -,h -CjH or h -CjH CfO) li-gand are irradiated in PPhj-containing solutions, they undergo simple carbonyl substitution and acyl decarbonylation, much as their monomeric counterparts do (see 13.2.4.1), e.g. ... 

The coupling constant is inconsistent with carboxyl coordination but consistent with carbonyl coordination 15). Similar data for -ketobu-tyrate 15) and oxalacetate (19) have been fit by exchange contributions (1/tm) and inner sphere contributions Tm and T2m)- The rates of formation of these metal bridge complexes from an outer sphere complex ( 3,4) are limited predominantly by the rate of dissociation of a water molecule from the coordination sphere of the enzyme-bound manganese (Figure 3, Table V) (15,19), as required by the Sj l-outer sphere mechanism of Eigen and Tamm (20),... 

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