The thermochemistry of Group 15 tetrachloride anions

Bond strengths for a series of Group 15 tetrachloride anions ACl 4 − (A = P, As, Sb, and Bi) have been determined by measuring thresholds for collision-induced dissociation of the anions in a flowing afterglow-tandem mass spectrometer. The central atoms in these systems have ten electrons, which violates the octet rule: the bond dissociation energies for ACl 4 − help to clarify the effect of the central atom on hypervalent bond strengths. The 0 K bond energies in kJ mol −1 are D(Cl 3A-Cl −) = 90 ± 7, 115 ± 7, 161 ± 8, and 154 ± 15, respectively. Computational results using the B3LYP/LANL2DZpd level of theory are higher than the experimental bond energies. Calculations give a geometry for BiCl 4 − that is essentially tetrahedral rather than the see-saw observed for the other tetrachlorides. NBO calculations predict that the phosphorus and arsenic systems have 3C–4E bonds, while the antimony and bismuth systems are more ionic.