Binuclear cobalt thiocarbonyl carbonyl derivatives: comparison with homoleptic binuclear cobalt carbonyls.

Density functional theory studies on Co(2)(CS)(2)(CO)(8) show the structure with two bridging CS groups to be the global minimum. Furthermore, swapping a terminal CO group with a bridging CS group to give a terminal CS group and a bridging CO group increases the energy of the structure by 7 +/- 2 kcal/mol. Thus, unbridged Co(2)(CS)(2)(CO)(8) structures lie at least 11 kcal/mol above the doubly bridged global minimum Co(2)(mu-CS)(2)(CO)(6), unlike Co(2)(CO)(8) where the doubly bridged and unbridged structures are significantly closer in energy. The lowest energy unsaturated Co(2)(CS)(2)(CO)(n) (n = 5, 4, 3) structures are predicted to contain four-electron donor bridging eta(2)-mu-CS groups, unlike the corresponding homoleptic carbonyls Co(2)(CO)(n+2), which contain only two-electron donor carbonyl groups. For example, the three lowest energy Co(2)(CS)(2)(CO)(5) structures contain a single eta(2)-mu-CS group accompanied by a Co-Co distance of approximately 2.7 A, consistent with the single bond required to give both cobalt atoms the favored 18-electron configuration.