Characteristics of novel sandwiched beryllium, magnesium, and calcium dimers: C5H5BeBeC5H5, C5H5MgMgC5H5, and C5H5CaCaC5H5

Resa, Carmona, Gutierrez-Puebla, and Monge recently reported the synthesis and characterization of a remarkable sandwiched di-zinc compound, Me5C5ZnZnC5Me5. Although Resa et al. emphasize that this is the first Zn(I) compound with a Zn–Zn bond, we find the CpMMCp sandwich metal dimer bonding motif to be even more fascinating. The present theoretical studies of CpBeBeCp, CpMgMgCp, and CpCaCaCp demonstrate that the structural pattern discovered for zinc is more generally applicable. The predicted Be–Be distance of 2.06 Å, Mg–Mg distance of 2.77 Å, and Ca–Ca distance of 3.74 Å are indicative of substantial Be–Be, Mg–Mg, and Ca–Ca bonding. The NBO analysis shows that each metal atom has one covalent bond (to the other metal atom) and a predominantly ionic linkage to the adjacent cyclopentadienyl ring.