Two different syntheses of N-heterocyclic carbene complexes of P^I cations are presented; the structural features of these phosphamethine cyanine salts, in conjunction with the results of computational investigations, provide insight into the nature of the bonding of these heavy allene analogues.

The structure of the phosphinyl radical, ṖR₂ [R = CH(SiMe₃)₂], has been determined by gas-phase electron diffraction (GED) together with ab initio molecular orbital calculations, and that of the corresponding diphosphine, (PR₂)₂, has been established by X-ray crystallography; the diphosphine behaves as an energy storage reservoir.