Butterfly and rhombus structures for binuclear cobalt carbonyl sulfur and phosphinidene complexes of the type Co2(CO)6E2 (E = S, PX).

Theoretical studies on Co(2)(CO)(6)(PX)(2) derivatives (X = H, Cl, OH, OMe, NH(2), NMe(2)) predict the lowest energy structures to be butterfly structures containing five two-electron two-center bonds in the central Co(2)P(2) unit. 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 Co(2)(CO)(6)(PX)(2) with only Co-P bonds in the Co(2)P(2) framework without any Co-Co or P-P bonds. In addition, for Co(2)(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 Co...Co bond. For the isoelectronic Co(2)(CO)(6)S(2) a rhombus structure is competitive in energy with the butterfly structures with five structures lying within approximately 4 kcal/mol thereby predicting a fluxional system. A tetrahedrane structure was not found for Co(2)(CO)(6)S(2) in contrast to the tetrahedrane structure known experimentally for the related Fe(2)(CO)(6)S(2) with one less electron per metal atom.