A DFT/electron localization function (ELF) study of the bonding of phosphinidenes with N-heterocyclic carbenes

The bonding between model phosphinidene PH and two types of cyclic Arduengo's carbenes, the aromatic C₃H₄N₂ (1) and the related saturated C₃H₆N₂ (2) species, respectively, have been analyzed in terms of the electron localization function (ELF). In a first step, the bare carbenes have been studied, and then, in a second step, the changes brought about by PH complexation have been treated. It has been shown that the bonding mode essentially results from a dative bond formed by the nucleophilic carbenic carbon of the ring and the PH acceptor site. Some back bonding from P creates a partial double-bond character around the C-P unit. The latter character is more pronounced when PH is linked to the saturated carbene 2. A comparison of the ring aromatic properties, in terms of nucleus-independent chemical shifts has been achieved, showing that complexation of 1 by PH diminishes the ring aromaticity through relocalization of the electrons on the N atoms.