Inorganic Niobium and Tantalum Octahedral Cluster Halide Compounds with Three-Dimensional Frameworks: A Review on Their Crystallographic and Electronic Structures

This review summarizes the development of the rich crystal and bonding chemistry of face-capped and edge-bridged inorganic niobium and tantalum octahedral cluster halide compounds, with a particular emphasis on those showing three-dimensional cluster frameworks. Discussion is made on varied structures and bonding which are intimately linked to the valence electron concentration, i.e., the number of electrons that held the octahedral metal cluster architecture. Exploration of the literature indicates that apart from Nb6I11 and derivatives, which show electron-deficient face-capped (M6X8X6a)-X-i units, compounds containing edge-bridged (M6X12X6a)-X-i motifs are the most largely encountered. Closed-shell compounds with a valence electron concentration of 16 are predominant, although a few 15-electron open-shell magnetic compounds or even 14-electron closed-shell species have also been reported. Particularly interesting from a structural point of view is the fashion in which these face-capped and edge-bridged clusters "pack" in crystals. The astonishing diversity of structural types, which are observed, is mainly due to the flexibility of the halogen ligands to coordinate in various manners to metal atoms. However, a rigorous structural analysis of these compounds reveals no close relationship between the valence electron concentration and the variability of the intercluster connections and/or the nature of the counterions. Indeed, the main bonding features of these compounds can be understood from the delocalized bonding picture of isolated "molecular-like" (M6X8X6a)-X-i or (M6X12X6a)-X-i clusters.