Characterizations of novel binuclear alkaline-earth metallocenes: M-2(eta(5)-E-5)(2) (M = Be, Mg and Ca; E = P and As)

A series of novel binuclear alkaline-earth metallocenes M-2(eta(5)-E-5)(2) (M = Be, Mg and Ca; E = P and As) have been studied at the B3LYP/6-311G* and BP86/6-311G* levels of density functional theory. The bonding between the two alkaline earth metal atoms and the bonding between the alkaline earth metal and the five-membered ring are investigated. Natural Bonding Orbital (NBO) analysis shows that the bonding between the metal atom and the five-membered ring is predominantly ionic and each metal atom is in its +1 oxidation state. The lighter alkaline earth metal has stronger bonding in both M-2(eta(5)-P-5)(2) and M-2(eta(5)-As-5)(2). A single metal-metal bonding exists between the two metal atoms. For the same alkaline earth metal, the metal-metal bond in M-2(eta(5)-As-5)(2) is weaker than that in M-2(eta(5)-P-5). Nucleus independent chemical shift (NICS) values confirm that the planar E-5(-) exhibits characteristics of aromaticity in these M-2(eta(n)-E-5)(2) species. The NICS values M-2(eta(5)-P-5)(2) are larger than those of the M-2(eta(5)-As-5)(2) analogues. The NICS(0.0) and NICS(1.0), values are 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).