Binuclear iron carbonyl nitrosyls: Bridging nitrosyls versus bridging carbonyls

The iron carbonyl nitrosyls Fe-2(NO)(2)(CO)(n) (n = 7, 6, 5, 4, 3) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods, for comparison of their predicted structures with those of isoelectronic cobalt carbonyl derivatives, The lowest energy structures for Fe-2(NO)(2)(CO)(7) and Fe-2(NO)(2)(CO)(6) have two NO bridges, and the lowest energy structure for Fe-2(NO)(2)(CO)(5) has a single NO bridge with metal-metal distances (BP86) of 3.161, 2.598, and 2.426 angstrom, respectively, corresponding to the formal metal-metal bond orders of zero, one, and two, respectively, required for the favored 18-electron configuration for the iron atoms. The heptacarbonyl Fe-2(NO)(2)(CO)(7) is thermodynamically unstable with respect to CO loss to give Fe-2(NO)(2)(CO)(6). The favored structures for the more highly unsaturated Fe-2(NO)(2)(CO)(4) and Fe-2(NO)(2)(CO)(3) also have bridging NO groups but avoid iron-iron bond orders higher than two by formal donation of five electrons from bridging NO groups with relatively short Fe-O distances. The lowest energy structures of the unsaturated Fe-2(NO)(2)(CO)(n) derivatives (n = 5, 4, 3) are significantly different from the isoelectronic cobalt carbonyls CO2(CO)(n+2) owing to the tendency for Fe-2(NO)(2)(CO)(n) to form structures with bridging NO groups and metal-metal formal bond orders no higher than two.