Theoretical studies on Co2(CO)6(PX)2 derivatives (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 Co2P2 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 Co2(CO)6(PX)2 with only Co–P bonds in the Co2P2 framework 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 Co⋯Co bond. For the isoelectronic Co2(CO)6S2 a 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)6S2 in contrast to the tetrahedrane structure known experimentally for the related Fe2(CO)6S2 with one less electron per metal atom.
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