Distinct π-Bonding Capability between Phosphinidene and Phosphonium Ion: A Computational Study

Abstract

The heavy dipnictenes (RE=ER, where E=P, As, Sb, and Bi with the substituent R) have essentially planar geometry and appreciable strength in pi-bonding, unlike related heavier main group 14 analogues of alkenes as concluded recently by Power. This work demonstrated that the protonated pnictenes behave more like the heavy carbene for their weak pi-bonding character from the computational study with the B3LYP/6-311++G** method. For example, although both phosphinidene (HP) and the phosphonium ion (H2P+) are isoelectronic to silylenes, the pi-bonding tendency of the former is rather strong and it forms a planar adduct with both the stable carbene and stable silylene ((HCNH)2E, where E=C and Si). In contrast, the latter forms trans-bent adducts with the two species. These results can be interpreted in terms of the Carter-Goddard-Malrieu-Trinquier (CGMT) model, and the fact that the value of DeltaEST [E(triplet)-E(singlet)] of the HP fragment increases significantly after protonation. All other heavy pnictenes resemble the phosphinidene. In contrast, nitrene (HN) and nitrenium (H2N+) have a ground triplet state, thus both have strong pi-bonding character similar to that of carbene.