Effect of π-donating substituents on the dative or covalent character of adducts of some simple “enium” ions with PMe3 and NMe3

The charge distribution, molecular structure and bond cleavage of formal donor adducts of phosphenium, arsenium, and the isolobal selenenyl cations with trimethylphosphane and trimethylamine have been explored by quantum chemical methods on Hartree–Fock, DFT and MP2 level. According to our results, adducts of methyl substituted cations are mainly covalent and therefore should be better described as phosphonium or ammonium ions. Switching from methyl substituted “enium” ions to their π-donor substituted analogs increases the dative character of the central bond to an extent for which a description as donor adducts of phosphenium or arsenium ions appears appropriate, while their selenium congener still remains preferentially covalent.