Binuclear nickel carbonyl thiocarbonyls: metal-metal multiple bonds versus four-electron donor thiocarbonyl groups

The structures of the mononuclear derivatives Ni(CS)(CO)(n) (n = 3, 2, 1, 0) and the binuclear derivatives Ni(2)(CS)(2)(CO)(n) (n = 5, 4, 3, 2) have been optimized by density functional theory for comparison with the corresponding structures of Ni(CO)(n+1) and Ni(2)(CO)(n+2), respectively. In the lowest energy structures for Ni(CS)(CO)(n) (n = 3, 2, 1), the nickel atom has approximate tetrahedral (n = 3), trigonal planar (n = 2), or bent coordination (n = 1) corresponding to 18-, 16-, and 14-electron metal configurations, respectively. The six lowest energy Ni(2)(CS)(2)(CO)(5) structures all have a single CE (E = S, O) bridge and a formal Ni-Ni single bond of length approximately 2.6 to approximately 2.7 A analogous to the lowest energy Ni(2)(CO)(7) structure. The Ni(2)(CS)(2)(CO)(5) structures with a bridging CS group are of lower energies than similar structures with a bridging CO group. Higher energy Ni(2)(CS)(2)(CO)(5) structures with a linear bridging CE group (E = O, S) and no Ni...Ni bond are also found with tetrahedral coordination for both nickel atoms. The lowest energy Ni(2)(CS)(2)(CO)(4) structures are doubly bridged structures with only two-electron donor CO and CS groups and Ni=Ni distances of approximately 2.5 A suggesting the formal double bond needed to give both nickel atoms the favored 18-electron configuration. For Ni(2)(CS)(2)(CO)(3) the structures with one four-electron donor bridging eta(2)-mu-CS group and a formal Ni=Ni double bond of approximately 2.4 to approximately 2.5 A are energetically preferred over triply bridged structures with a shorter Ni[triple bond]Ni distance of approximately 2.2 A, corresponding to a formal triple bond and similar to the lowest energy Ni(2)(CO)(5) structure. The lowest energy Ni(2)(CS)(2)(CO)(2) structures are generally derived from Ni(2)(CS)(2)(CO)(3) structures by removal of a carbonyl group. No formal quadruple bonds are found in any of the Ni(2)(CS)(2)(CO)(2) structures, as indicated by the absence of ultrashort Ni(-4)Ni distances.