Metal clusters isolobal analogy

The addition of transition metal fragments ML (L = two-electron donor ligand) across formally unsaturated metal-metal or metal-carbon bonds is a well-developed synthetic route to heteronuclear clusters (1,2,11,12,27) and has received theoretical justification from Hoffmann s isolobal principle (46). The addition of a PtL2 fragment across an M=M double bond may be considered as analogous to the reaction of a carbene with an olefin, resulting in a cyclopropane. The use of isolobal analogies in the directed synthesis of heteronuclear clusters has been reviewed (11,12,27). 

There are still only a few examples of mixed metal-transition metal clusters containing boron atoms. The first to be reported was Cp2Co2Fe(CO)4B3H3 (92). This cluster results from the photolysis of 2-CpCoB4Hg [a basally substituted isolobal analog of pentaborane(9)] with Fe(CO)5. NMR spectroscopic data are consistent with the proposed structure shown in Fig. 17. 

Exercise 4.2. What hydrocarbon cluster is analogous to Co4(CO)i2 Write the main-group and metal isolobal fragments. Write out the compositions and draw the structures of all the mixed Co/C clusters possible between these two homonuclear Co and C clusters. 

Additional tetrahedrane clusters are formed when vertices in Co4(CO)i2 are substituted with isolobal (see Isolobal Analogy) (or nonisolobal) metal fragments. In this category are the mixed Co-Rh and Co-Ir carbonyl complexes, which can be formed from the reaction of M4(CO)i2 (M = Rh, Ir) and Co2(CO)g. For example, Co3Rh(CO)i2, which has been used as a hydroformylation catalyst precursor, is prepared in this way. Furthermore, since the Co(CO)3 group is isolobal... 

Theoretical work over the last 15 years has done much to advance our understanding of structure and bonding in metal clusters. The isolobal analogy " ... 

THE ISOLOBAL ANALOGY AND METAL-METAL BONDING IN ORGANOMETALLIC CLUSTERS... 

The isolobal analogy [9,10] tells us that this n + 1)-SEP rule can be in principle extended to any type of closo skeleton made of fragments possessing a similar set of 3 FOs (see Fig. 1), irrespectively of their energy and precise shape or composition. This is indeed the case of many transition-metal carbonyl or phosphine organome-tallic clusters, most of them built from conical ML3 fragments. For example, the three octahedral clusters [BeHe], [Ru6(CO)ig] [46], and [Ru4(CO)i2Bi2] [47] are 7-SEP species. ... 

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