Isoelectronic cluster isolobal

Fig. 28. Substitution of isolobal and isoelectronic CH and M(CO)3 (M = Co or Ir) fragments in tetrahedral clusters. It is interesting that the final member of the series C4H4 (tetrahedrane) has not been isolated.

The frontier orbitals of Fe(CO)3 and of CoCp are isolobal with those of B—H (p. 207). They consist of a radial hybrid directed to the centre of the polyhedron and two tangential orbitals at right angles to each other. In the Fe(CO)3 fragment (or CoCp fragment) these orbitals, as in B—H, contain two valence electrons. Fe(CO)3 can therefore replace B—H in deltahedral clusters. Mn(CO)3 provides one less valence electron than Fe(CO)3, so that one extra electron is required from the remaining cluster atoms. With Co(CO)3 one electron less is needed for an isoelectronic arrangement. 

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