Binuclear cyclopentadienylmetal nitrosyls of iron, cobalt, and nickel: comparison with related carbonyl derivatives.

The binuclear cyclopentadienylmetal nitrosyls and carbonyls Cp2M2(AO)n (A = N, M = Fe, Co, Ni; A = C, M = Ni; n = 2, 1) are studied by density functional theory using the B3LYP and BP86 functionals. In general, structures with bridging AO ligands are energetically preferred over those with terminal AO ligands. Thus, the global minima for Cp2M2(AO)2 are all found to have closely related axial dimetallocene structures with two symmetrically bridging AO ligands but variable planarity of the central M(mu-AO)2M units. Similarly, the single AO ligands in the global minima for Cp2M2(AO) are found to bridge symmetrically the pair of metal atoms. However, structures with terminal AO groups and a single bridging Cp ligand are also found at accessible energies for CpM2(NO)(mu-Cp) (M = Fe and Co) and CpNi2(CO)(mu-Cp). The metal-metal bond distances in Cp2M2(AO)n derivatives correlate reasonably well with the requirements of the 18-electron rule. In this connection, the unusual dimer Cp2Ni2(mu-NO)2 has a Ni-Ni bond distance suggestive of a single bond and geometry suggesting one one-electron donor bridging NO group and one three-electron donor bridging NO group. However, dissociation of Cp2Ni2(mu-NO)2 into the well-known stable monomer CpNiNO is highly favored energetically.