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, M2(η5-E5)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 M2(η5-Sb5)2 and M2(η5-Bi5)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 M2(η5-E5)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 M2(η5-Sb5)2 and M2(η5-Bi5)2 species. Nucleus-independent chemical shifts (NICS) values suggest the planar E5- exhibits characteristics of aromaticity in these M2(η5-E5)2 species. All the NICS values decrease in the order of Be2(η5-E5)2 > Mg2(η5-E5)2 > Ca2(η5-E5)2 except for the NICS (1.0) values of the M2(η5-Bi5)2 (M = Be, Mg, Ca) species. The absolute values of NICS (0.0), NICS (0.5) and NICS (1.0) for M2(η5-Sb5)2 are larger than those of the corresponding M2(η5-Bi5)2.