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

Abstract A series of all-metal dinuclear alkaline-earth metallocenes, M 2 (η 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 (η 5 -Sb 5 ) 2 and M 2 (η 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 (η 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 (η 5 -Sb 5 ) 2 and M 2 (η 5 -Bi 5 ) 2 species. Nucleus-independent chemical shifts (NICS) values suggest the planar E 5 - exhibits characteristics of aromaticity in these M 2 (η 5 -E 5 ) 2 species. All the NICS values decrease in the order of Be 2 (η 5 -E 5 ) 2  > Mg 2 (η 5 -E 5 ) 2  > Ca 2 (η 5 -E 5 ) 2 except for the NICS (1.0) values of the M 2 (η 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 (η 5 -Sb 5 ) 2 are larger than those of the corresponding M 2 (η 5 -Bi 5 ) 2 .