Ab initio MO Studies on the Coordination Bond in Mercury(II)-Ethylene Complexes and Nucleophilic Reaction at the Coordinated Ethylene Moiety

Abstract

An ab initio MO study was carried out on the bonding in [Li(C₂H₄)]⁺ and [HgH(C₂H₄)]⁺, and on the H₂O attack against these ethylene ligand. The effective core pote n tial of Topiol et al. was used for Hg and the usual 3-21 G basis set was employed for the ligand a t oms. The ethylene coordination in [Li(C₂H₄)]⁺ receives considerable stabilization from e l e ctrostatic interaction, and the ethylene coordination in [HgH(C₂H₄)]⁺ has larger donative in teraction than back donative interaction. Both considerably donative and back donative in teractions are contributing to stabilization in [PtCl₃(C₂H₄)]^-. The H₂O attack on free et hylene molecule results in remarkable destabilization of the reaction system. The H₂O app roach to [Li(C₂H₄)]⁺ presents energy stabilization at the H₂O-C distance of 2.5∼3.OÅ, whereas th e energy is destabilized at the shorter H2O-C distance. Thus, the H2O attack on [Li(C₂H₄)]⁺ is concluded to be difficult. The H₂O approach to [HgH(C₂H₄)]⁺ causes the energy stabilization at H₂O-C distance of 2.5∼1.5Å., suggesting that this nucleophilic reaction is facile and the H₂O molecule can approach the C₂H₄ moiety to make almost the C-O covalent bond with little barrier. The ease of the reaction can be explained in terms of the orbital mixing between ethylene π, π^*, and H₂O HOMO.