Comparison of pentavalent with trivalent pnicogen bonding

DFT calculations are applied to the interactions between a series of MeZX_n 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.