Steve Scheiner
DFT calculations are applied to the interactions between a series of MeZXn Lewis acids (Z = Sb, As, P, X = H, F, n = 2,4) and NH3 as universal Lewis base. The trivalent species form classic noncovalent pnicogen bonds that are strengthened by a heavier Z and by electron-withdrawing substituents. The interaction energies are closely correlated with the value of the maximum in the molecular electrostatic potential at the Z σ-hole. Raising the number of substituents to five strengthens the pnicogen bonding, with the interaction energies of MeZH4 roughly triple those of MeZH2. An even larger increment occurs in the fluorinated cases. The Z⋯N bonds formed by MeZF4 are much shorter, with interaction energies around 40 kcal mol-1. These bonds would be better characterized as covalent, as further supported by internal bond lengths and AIM analysis. Replacement of the methyl group by electron-withdrawing o-carborane or F amplifies the bond strengths, while electron-donating amino weakens the bonding to the point that pentavalent NH2PH4 has nearly lost its pnicogen bonding ability.