Low-valent alkaline-earth metals in all-metal dinuclear metallocenes M2(η5-E5)2 (M = Be, Mg, Ca; E = Sb, Bi)

A series of all-metal dinuclear alkaline-earth metallocenes, M-2(eta(5)-E-5)(2) (M = Be, Mg, Ca; E = Sb, Bi), have been calculated by the density functional theory (DFT). Natural Bonding Orbital (NBO) analysis indicates that the metal-metal bonds of the title compounds are all single bonds with each metal in its +1 oxidation state, and the bonding between the metal and the all-metal ligand is mainly ionic. For both M-2(eta(5)-Sb-5)(2) and M-2(eta(5)-Bi-5)(2), the metal-ligand bonds are very strong and the lighter alkaline-earth metal has stronger metal-ligand bonding. By comparing the dissociation energies of metal-metal bond and metal-ligand bond for each M-2(eta(5)-E-5)(2), the single metal-metal bond is much weaker than the metal-ligand bond with the same metals. The Mg-Mg bond is the strongest in the M-2(eta(5)-Sb-5)(2) and M-2(eta(5)-Bi-5)(2) species. Nucleus-independent chemical shifts (NICS) values suggest the planar E-5(-) exhibits characteristics of aromaticity in these M-2(eta(5)-E-5)(2) species. All the NICS values decrease in the order of Be-2(eta(5)-E-5)(2) > Mg-2(eta(5)-E-5)(2) > Ca-2(eta(5)-E-5)(2) except for the NICS (1.0) values of the M-2(eta(5)-Bi-5)(2) (M = Be, Mg, Ca) species. The absolute values of NICS (0.0), NICS (0.5) and NICS (1.0) for M-2(eta(5)-Sb-5)(2) are larger than those of the corresponding M-2(eta(5)-Bi-5)(2). (C) 2010 Elsevier B.V. All rights reserved.