Metal-metal bonding in binuclear metal carbonyls with three bridging methylaminobis(difluorophosphine) ligands: iron, cobalt, and nickel derivatives.

The [CH3N(PF2)2]3M2(CO)n (M = Fe, Co, Ni; n = 2, 1, 0) derivatives, in which a metal-metal bond is bridged by three CH3N(PF2)2 groups similar to the known very stable [CH3N(PF2)2]3Co2(CO)2, have been investigated by density functional theory. The lowest energy structures for the dicarbonyls [CH3N(PF2)2]3M2(CO)2 (M = Ni, Co, Fe) are predicted to be closed-shell singlets with metal-metal distances of approximately 3.94 A, approximately 2.80 A, and approximately 2.63 A, respectively, corresponding to the metal-metal bonds of orders 0, 1, and 2, required to give both metal atoms the favored 18-electron configurations. For the monocarbonyls [CH3N(PF2)2]3M2(CO) (M = Ni, Co, Fe), unsymmetrical structures with a carbonyl group terminally bonded to one metal are greatly preferred energetically over structures in which the carbonyl group bridges the pair of metal atoms. The lowest energy structures for [CH3N(PF2)2]3M2(CO) (M = Ni, Co, Fe) are singlet, triplet, and singlet states, respectively, with metal-metal distances of approximately 2.77 A, approximately 2.38 A, and approximately 2.13 A, respectively. These distances correspond to the metal-metal dative bonds of orders 1, 2, and 3, required to give both metal atoms the favored 18-electron configuration. Among the carbonyl-free species the iron compound [CH3N(PF2)2]3Fe2 is of particular interest since its very short Fe[double double bonds]Fe distance of approximately 2.02 A suggests the formal quadruple bond required to give both iron atoms the favored 18-electron configuration.