Stabilization of binuclear chromium carbonyls by substitution of thiocarbonyl groups for carbonyl groups: nearly linear structures for Cr(2)(CS)(2)(CO)(9).

The chromium carbonyl thiocarbonyls Cr(CS)(CO)(n) (n = 5, 4, 3) and Cr(2)(CS)(2)(CO)(n) (n = 9, 8, 7, 6) were studied by density functional theory (DFT). The expected octahedral structure was found for the known Cr(CS)(CO)(5). The structures for the unsaturated derivatives Cr(CS)(CO)(n) (n = 4, 3) are derived from the octahedral Cr(CS)(CO)(5) by removal of one or two carbonyl groups, respectively. The lowest energy structures for the binuclear derivatives Cr(2)(CS)(2)(CO)(n) (n = 9, 8, 7, 6) all contain four-electron donor bridging eta(2)-mu-CE (E = O, S) groups. For the formally saturated Cr(2)(CS)(2)(CO)(9), no chromium-chromium bond is then required to give the chromium atoms the favored 18-electron configuration. This leads to a uniquely linear Cr-C-O-->Cr arrangement or bent Cr-C-S-->Cr arrangement (C-S-->Cr angle of approximately 110 degrees ) with a long clearly nonbonding Cr...Cr distance. A similar structural feature is found in the known stable arene-chromium carbonyl thiocarbonyl (eta(6)-MeC(6)H(5))Cr(CO)(2)[CS-->Cr(CO)(5)]. The lowest energy structures for the formally unsaturated Cr(2)(CS)(2)(CO)(n) (n = 8, 7, 6) are predicted to have one (n = 8) or two (n = 7, 6) four-electron donor eta(2)-mu-CS groups with a Cr-Cr single bond (n = 8 and 7) or Cr horizontal lineCr double bond (n = 6) to give both chromium atoms the favored 18-electron configuration. The lowest energy structures for the binuclear Cr(2)(CS)(2)(CO)(n) (n = 9, 8, 7, 6) are all predicted to be stable with respect to fragmentation into mononuclear Cr(CS)(CO)(m) in contrast to the homoleptic Cr(2)(CO)(11). This suggests that there is a reasonable chance that at least some of the binuclear Cr(2)(CS)(2)(CO)(n) (n = 9, 8, 7, 6) derivatives will be synthesized as stable or at least detectable molecules.