DIVERSITY OF BONDING IN METHYL ATE ANIONS OF THE FIRST-ROW AND 2ND-ROW ELEMENTS

CCSD/6-311++G(2d,2p) quantum-chemical calculations carried out in conj unction with rigorous analysis of the computed electronic wave functio ns 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 wit h ionic bonds between the central atom and the ligands, formed by elem ents of the first three main groups, are very stable to the loss of CH 3-. Hypervalent anions, which obtain from the other second-row element s, possess largely covalent X-C bonding that provides them with (somet imes marginal) stability to the ligand loss. The other two types of at e anions are unique to the first-row elements. The ''double-Rydberg'' species are derived from nitrogen and oxygen. They are unstable to ele ctron loss and thus most probably not observable in the gas phase. The potentially observable (CH3)(2)F- species is the only member of its c lass. It has a positive vertical ionization potential, and its dissoci ation into CH3F and CH3- is predicted to proceed through a small barri er. This anion is found to possess a peculiar electronic structure tha t involves an entirely new type of bonding, namely, a bypass linkage o f the methyl ligands.