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Trinuclear iron carbonyl thiocarbonyls: the preference for four- and six-electron donor bridging thiocarbonyl groups over metal-metal multiple bonding, while satisfying the 18-electron rule.

Theoretical studies on Fe(3)(CS)(3)(CO)(9) show that the structure having an isosceles Fe(3) triangle with one edge bridged by two CS groups analogous to the Fe(3)(CO)(10)(mu-CO)(2) structure of Fe(3)(CO)(12) is energetically favored over structures of other types. However, the Fe(3)(CS)(3)(CO)(9) system is very highly fluxional with five distinct equilibrium structures lying within 6 kcal/mol of this global minimum. The lowest energy structures predicted for the unsaturated Fe(3)(CS)(3)(CO)(n) (n = 8, 7, 6) are very different from those previously predicted for the corresponding homoleptic carbonyls Fe(3)(CO)(n+3). Thus Fe(3)(CS)(3)(CO)(n) (n = 8, 7, 6) structures with four- and six-electron donor thiocarbonyl groups and only formal Fe-Fe single bonds are energetically preferred over structures with some iron-iron multiple bonding. For Fe(3)(CS)(3)(CO)(8) the lowest energy structures have a unique four-electron donor thiocarbonyl group bridging all three iron atoms. Similarly, for Fe(3)(CS)(3)(CO)(7) the lowest energy structures have a unique six-electron donor thiocarbonyl group bridging all three iron atoms similar to the remarkable six-electron donor carbonyl group in the known stable Cp(3)Nb(3)(CO)(6)(eta(2)-mu(3)-CO). For the even more unsaturated Fe(3)(CS)(3)(CO)(6), the lowest energy structures have both a six-electron donor thiocarbonyl group bridging all three iron atoms and a four-electron donor thiocarbonyl group bridging one of the Fe-Fe edges. Thus all of these structures of the unsaturated derivatives Fe(3)(CS)(3)(CO)(n) (n = 8, 7, 6) require only formal Fe-Fe single bonds for each iron atom to have the favored 18-electron configuration. From the wide range of formal Fe-Fe single bonds found in these structures the lengths of doubly bridged single bonds are seen to be approximately 2.5 to 2.6 A whereas unbridged single bonds are significantly longer at approximately 2.7 to 2.8 A.