Structure and electron counting of tetrahedral and butterfly M2E2 clusters: an Mo analysis of (LnM)2(μ-X2) (X  P, PR, PM′L5) complexes

Abstract

M2E2 clusters where both M and E are in a local conical coordination adopt a tetrahedral structure if they have six skeletal electron pairs (SEP). For a 5 SEP count, two different geometries are observed: a “contracted” tetrahedron such as Fe2(CO)6(μ-C2-t-Bu)2 or a butterfly structure such as Fe2(CO)6[μ-PMnCp(CO)2]2. By means of extended Hückel calculations on Co2(CO)6E2 (E  P, PH, PCr(CO)5, CH, CO) complexes, the criteria determining the choice of one of these two geometries are analysed. In particular, the role of the energy of the π-type frontier orbitals of E is emphasized. The possibility of existence of tetrahedral and butterfly-like M2E2 clusters is examined for 5, 6, and 7 SEP counts.