Predictive concept for lone-pair distortions - DFT and vibronic model studies of AXn-(n-3) molecules and complexes (A = NIII to BiIII; X = F-I to I-I; n = 3-6)

The stereochemical and energetic consequences of the lone-pair effect in the title molecules and complexes have been studied by DFT calculations based on a vibronic coupling concept. The anionic complexes were examined as bare entities and, more realistically, in a polarizable charge-compensating solvent continuum. The tendency for distortions of AX3 compounds away from the high-symmetry parent geometry becomes more pronounced the larger the chemical hardness of a molecule and its constituents is; on the other hand, anionic complexes AXn-(n-3) (n = 4-6) become softer and less susceptible to distortion as compared to the corresponding AX3 molecule, the larger the coordination number and the anionic charge are. Thus, while all AX(3) compounds adopt the distorted C3v structure, only very few AX6(3-) species are calculated to deviate from the parent Oh geometry. If a complex possesses a low stabilization energy due to an unfavorable central ion/ligand size ratio, vibronic coupling may even lead to complete dissociation of one (SbF6(3-) --> SbF5(2-) + F-) or more (PF6(3-) --> PF4- + 2F-) ligands. The derived hardness rule perfectly covers the reported structural findings. The calculations indicate that the lone-pair effect is an orbital overlap phenomenon. The interpair repulsion within the valence shell, keeping the average bond distances constant, does not stabilize the distorted with respect to the parent geometry, in disagreement with the VSEPR model.