Theoretical investigations of the chemical bonding in MM'O 2 clusters (M, M' = Be, Mg, Ca).

Motivated by the known stability of the somewhat unusual Be2O2 rhombus, which features a short Be-Be distance but no direct metal-metal bonding, we investigate the nature of the bonding interactions in the analogous clusters MM'O-2 (M, M' = Be, Mg, Ca). CCSD/cc-pVTZ and CCSD(T)/cc-pVQZ calculations, amongst others, are used to determine optimized geometries and the dissociation energies for splitting the MM'O-2 clusters into metal oxide monomers. The primary tools used to investigate the chemical bonding are the analysis of domain-averaged Fermi holes, including the generation of localized natural orbitals, and the calculation of appropriate two- and three-center bond indices. Insights emerging from these various analyses concur with earlier studies of M2O2 rhombic clusters in that direct metal-metal bonding was not observed in the MM'O-2 rings whereas weak three-center (3c) bonding was detected in the MOM' moieties. In general terms, these mixed MM'O-2 clusters exhibit features that are intermediate between those of M2O2 and M'O-2(2), and the differences between the M and M' atoms appear to have little impact on the overall degree of 3c MOM' bonding.