σ–σ and σ–π pnicogen bonds in complexes H₂XP:PCX, for X = F, Cl, OH, NC, CN, CCH, CH₃, and H

Abstract

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 H₂XP_s:P_tCX 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 π*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 σ orbital to the π*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.