A G2 Ab Initio Investigation of Ligand-Exchange Reactions Involving Mono- And Bis-Adducts of the Phosphenium Ion

Ab initio calculations at the G2 level have been employed to investigate the ligand-exchange reactions between mono-adducts of the phosphenium ion (e.g., [H3N-PH2]+) and simple first- or second-row Lewis bases (e.g., NH3). We have found virtually all the reactions to proceed without an intermediate barrier via a bis-adduct of the phosphenium ion with two Lewis bases (e.g., [H3N-PH2-NH3]+). The ligand-exchange reactions are predicted to be experimentally feasible and the bis-adducts of the phosphenium ion to be experimentally observable in appropriate cases. The energetics of the ligand-exchange reactions may be rationalized using qualitative orbital interaction arguments in terms of the electronegativity of the ligand in the mono-adducts and the donor ability of the reacting Lewis base. Comparisons with previous investigations of corresponding ligand-exchange reactions involving π-ligands reveal that the electronic characteristics of the bis-adduct resemble those of the transition structures for π-ligand exchange.