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. 

The cluster ResIn4(CO)32 has the structure shown below (tetracapped tetrahedron). Justify its geometry based on the isolobal analogy and one form of the electron-counting rules. 

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... 

Early work in this area was often serendipitous but as the field has developed, underpinned concepts such as cluster Electron Counting rules and Hoffman s Isolobal Analogy, rational cluster syntheses have begun to appear. ... 

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

Chapter 13 in the first edition covered a variety of topics relating to applications of organometallic chemistry to other areas of science. This material has been eliminated with the exception of material on fullerenes, which now appears in Chapter 6. Chapter 13 updates the discussion of the isolobal analogy and cluster chemistry, which was the purview of Chapter 12 in the first edition. 

Two classes of complexes are derived from Co4(CO)i2 by replacement of one or two Co(CO)3 groups by the isolobal (see Isolobal Analog RC unit, leading to cluster types (RC=CR)Co2 (CO)e (7) and RCCo3(CO)g (8). Both classes of compounds have an abundant variety of derivatives with widely differing R groups and hetero functions, and they exhibit a rich and extended chemistry (Scheme 8). 

---