Influence of Internal Angular Arrangement on Pnicogen Bond Strength

The three Z-X covalent bonds of a ZX(3) unit (Z = P, As, Sb, Bi) are normally arranged in a pyramidal structure. Quantum chemical calculations show that pnicogen bonds (ZBs) to the central Z are weakened if ZX(3) is flattened, as in the opening of an umbrella. The partial closing of the umbrella has the opposite effect of substantially strengthening these ZBs, even amounting to a 2- or 3-fold magnification in certain cases. The strongest such bonds, wherein Sb and Bi are in a strained configuration within a ZO(3)CH model system, have interaction energies of 20 kcal/mol with an NH3 base. Most of these systems, whether flattened or more pyramidal, are capable of engaging in three ZBs simultaneously, despite a certain amount of negative cooperativity.