THEORETICAL-STUDY OF ONE AND 2-AMMONIA MOLECULES BOUND TO THE 1ST-ROW TRANSITION-METAL IONS

The binding energies of MNH3+ and M(NH3)2+ where M = (Sc-Cu) are studied using the modified coupled-pair functional (MCPF) approach. These results are compared with experiment and with previous comparable calculations performed for the water and diwater systems. In addition to the electrostatic interaction, other factors such as 4s4p or 4s3d-sigma hybridization and 4s to 3d promotion on the metal atom contribute to determining the ground state and the magnitude of the binding energy. All of the M(NH3)2+ systems have linear NMN ground-state structures. However, for Mn(NH3)2+, the lowest septet state has a bent structure that is only 1 kcal/mol higher than the 5A1g linear D3d ground state. In contrast, Mn(H2O)2+ has a bent 7A1 ground state. This difference is probably due to the greater electrostatic binding in the ammonia systems, which favors the low-spin linear state due to its much shorter bond length. We find the second ligand binding energy to be less than the first except for Cr+, Fe+, and Co+. Overall the theoretical binding energies agree with experiment to within about 6 kcal/mol, except for the first ligand binding energy for V+ and both ligand binding energies for Co+.