Butterfly and rhombus structures for binuclear cobalt carbonyl sulfur and phosphinidene complexes of the type Co2(CO)6E2(E = S, PX)Electronic supplementary information (ESI) available: Tables S1–S7: The vibrational frequencies for the structures of Co2(CO)6E2(E = S, PH, PCl, POH, POMe, PNH2or PNMe2) at the B3LYP/6-311G(d) and BP86/6-311G(d) levels; Tables S8–S14: The Cartesian coordinates of the optimized Co2(CO)6E2(E = S, PH, PCl, POH, POMe, PNH2or PNMe2) structures at the B3LYP/6-311G(d) and BP86/6-311G(d) levels; complete Gaussian 03 reference (ref. 42). See DOI: 10.1039/b913117e

Theoretical studies on Co2(CO)6(PX)2derivatives (X = H, Cl, OH, OMe, NH2, NMe2) predict the lowest energy structures to be butterfly structures containing five two-electron two-center bonds in the central Co2P2unit. Among these butterfly structures the energy increases as the unique bond forming the “body” of the butterfly changes from Co–Co to Co–P and then P–P. Higher energy rhombus structures are also found for Co2(CO)6(PX)2with only Co–P bonds in the Co2P2framework without any Co–Co or P–P bonds. In addition, for Co2(CO)6(POR)2(R = H, Me) still higher energy “diphosphine” structures are also found containing only three rather than four Co–P bonds, one P–P bond, and no CoCo bond. For the isoelectronic Co2(CO)6S2a rhombus structure is competitive in energy with the butterfly structures with five structures lying within ∼4 kcal/mol thereby predicting a fluxional system. A tetrahedrane structure was not found for Co2(CO)6S2in contrast to the tetrahedrane structure known experimentally for the related Fe2(CO)6S2with one less electron per metal atom. [ABSTRACT FROM AUTHOR]