Sigma-hole-supported interactions in complexes of group 5 oxyhalides (MOX₃) with insights for their extended solids

The existence and nature of sigma hole type interactions in inorganic extended solids remain largely unexamined, even though the influence of such interactions for heteroorganic compounds is well known. The series of group 5 MOX₃ oxyhalides are intriguing in this regard since several of the known crystal structures are molecular solids with highly polarized M centers. We examine computationally the bonding in the isolated MOX₃ molecules and their smallest clusters (MOX₃)_n where M = V, Nb, and Ta, and X = H, F, Cl, Br, and I. That investigation provides us with substantial evidence, based on patterns and quantitative trends exhibited in the bonding and energetics of the oxyhalide clusters, that several of the extended solids are stabilized by sigma-hole-supported M⋯O and more rarely M⋯X bonding interactions between the MOX₃ molecules or dimers. In particular, one-dimensional stacks of doubly bridged, X₂OMX₂MOX₂, dimer units in several of the oxyhalide crystal structures are shown to be propagations of sigma-hole-supported bonding interactions with weak charge transfer contributions that are already established at the level of the tetramer. Our results are in harmony with a more recent crystal structure for NbOCl₃ over a much older proposal.