Tetrahedrane isolobal analogs

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... 

The analogies are far from perfect. For example, CH dimerises to give acetylene HC CH but the isolobal Co(CO)3 does not dimerise. However, Co(CO)3 does behave as a trivalent radical. The cluster carbonyl Co4(CO)12 (Fig. 8.8) can be seen as analogous to tetrahedrane C4H4 (note, however, that three of the CO groups are bridging). The compounds Co (CO)3 (CR)4 n = 1,2,3) have been prepared, all containing tetrahedral Co C4 clusters. 

Since the W(C0)2(O-CsHs) fragment is isolobal with RC5 (23), addition of [w=CC6HifMe-4(C0)2(ti-CsHs)] across the C02 and Rh2 dimetal centres of [Co2(C0)s] and [Rh2(y-CO)2(n-CsMes)2] to give compounds (40) and (41), respectively, has also been successfully accomplished(21). Formation of (40) and (41) in this manner is analogous to the reaction of alkynes with these and other dimetal complexes affording tetrahedrane-type structures with a dicarbadimetalla core. 

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