Diversity of bonding in methyl ate anions of the first- and second-row elements

CCSD/6-311++G(2d,2p) quantum-chemical calculations carried out in conjunction with rigorous analysis of the computed electronic wave functions reveal the existence of four different types of methyl ate anions (CH3)(n+1)X-, where X is a first- or second-row element. Ate anions with ionic bonds between the central atom and the ligands, formed by elements of the first three main groups, are very stable to the loss of CH3-. Hypervalent anions, which obtain from the other second-row elements, possess largely covalent X-C bonding that provides them with (sometimes marginal) stability to the ligand loss. The other two types of ate anions are unique to the first-row elements. The "double-Rydberg" species are derived from nitrogen and oxygen. They are unstable to electron loss and thus most probably not observable in the gas phase. The potentially observable (CH3)(2)F- species is the only member of its class. It has a positive vertical ionization potential, and its dissociation into CH3F and CH3- is predicted to proceed through a small barrier. This anion is found to possess a peculiar electronic structure that involves an entirely new type of bonding, namely, a bypass linkage of the methyl ligands.