Structural changes upon replacing carbonyl groups with thiocarbonyl groups in first row transition metal derivatives: new insights.

The chemistry of metal thiocarbonyls is much more limited than that of metal carbonyls because of the instability of CS as a synthetic reagent. In view of the many gaps remaining in experimentally realized metal thiocarbonyl chemistry, theoretical studies using density functional methods have been used to explore the possible future scope of metal thiocarbonyl chemistry. This paper reviews such theoretical studies on binuclear metal carbonyl derivatives of the types M(2)(CS)(2)(CO)(n) and Cp(2)M(2)(CS)(2)(CO)(n) (Cp = η(5)-C(5)H(5); M = V through Ni) as well as the trinuclear and tetranuclear iron carbonyls Fe(3)(CS)(3)(CO)(n) (n = 9, 8, 7, 6) and Fe(4)(CS)(4)(CO)(n) (n = 12, 11, 10, 9). The substitution of one or two CO groups with CS groups to give less symmetrical structures leads to many more isomers. Structures in which a four-electron donor thiocarbonyl group uses its sulfur atom to bridge a metal-metal bond as a η(2)-μ-CS ligand are more favorable in binuclear metal thiocarbonyl chemistry than corresponding structures in metal carbonyl chemistry owing to the basicity of the sulfur atom. Six-electron donor thiocarbonyl groups bridging clusters of three or four iron atoms are also found in low-energy structures including a particularly favorable Fe(4)(CS)(4)(CO)(10) structure suggested as a possible target for future synthetic chemistry. In thiocarbonyl substitution products of simple binuclear metal carbonyls such as Fe(2)(CO)(9) [= Fe(2)(CO)(6)(μ-CO)(3)], Co(2)(CO)(8) [= Co(2)(CO)(6)(μ-CO)(2)], and Cp(2)Fe(2)(CO)(4) [= Cp(2)Fe(2)(CO)(2)(μ-CO)(2)], structures with bridging CS groups are invariably lower energy structures than isomeric structures with bridging CO groups.