σ–σ and σ–π pnicogen bonds in complexes H2XP:PCX, for X = F, Cl, OH, NC, CN, CCH, CH3, and H
Ab initio MP2/aug’-cc-pVTZ calculations have been carried out on complexes H2XPs:PtCX, for X = F, Cl, OH, NC, CN, CCH, CH3, and H, in search of complexes stabilized by P···P pnicogen bonds. These intermolecular bonds arise when a pnicogen atom acts as a Lewis acid for complex formation. Three sets of equilibrium structures have been found on the H2XPs:PtCX potential surfaces. Conformation A complexes have P···P σ–σ pnicogen bonds, which involve the σ systems of both P atoms. Conformations B and C are stabilized by σ–π pnicogen bonds, which involve the σ system of H2XP and the π system of PCX. Binding energies of B and C complexes are similar and are greater than the binding energies of the A conformers. Charge transfer stabilizes A, B, and C conformers. In A complexes, the dominant charge transfer is from the lone pair of PCX to the antibonding σ*P–A orbital of PH2X, with A the atom of X directly bonded to P. For conformations B and C, the dominant charge transfer is from the P=C π orbital to the σ*P–A orbital of H2XP. Although the binding energies of these complexes do not correlate with the intermolecular P–P distances, both the charge-transfer energies and the equation-of-motion coupled cluster singles and doubles one-bond 31P–31P spin–spin coupling constants do correlate with the P–P distances. The largest coupling constants 1pJ(P–P) are found for complexes with conformation A, due to the nature of the σ–σ pnicogen bond and the dominance of the Fermi contact term. For a given X, 1pJ(P–P) values are ordered A > C > B.