The stability and eventual lone pair distortion of the hexahalide complexes and molecules of the fifth to eighth main-group elements with one lone pair, as isolated entities or in oligomeric clusters: a vibronic coupling and DFT study

The introduced DFT-supported vibronic coupling model together with the hardness rule indicates, for the title compounds, that the tendency toward lone pair (LP) distortions decreases with increasing coordination number (CN) and upon proceeding from fluoride to iodide as the ligands. Thus, only some hexafluoro complexes and molecules are calculated to actually undergo LP deformations; here, from the possible highest-symmetry deformations, those with C(4v) geometry possess the lowest energy, leading to the complete ablation of one ligand and, hence, explaining the nonexistence of the complexes AsF6(3-), SbF6(3-), and SF6(2-). If a lower-symmetry strain is imposed on the octahedral species, for example, induced by the simultaneous presence of terminal and bridging ligands in oligomers, the vibronic energy potential is activated. It may induce pronounced distortions, which are much larger than those of analogous clusters with central ions lacking the LP. Dimers and tetramers with common edges and faces are investigated, with the predicted and calculated deformations of the constituting octahedra again following the stability sequence C(4v) > C(2v) > C(3v). The model nicely accounts for the observed trends, as well as reproduces the experimental structures and energy balances, at least semiquantitatively; its predictive power exceeds that of the valence-shell electron-pair repulsion concept.