Lattice strains in the ligand framework in the octahedral metal cluster compounds as the origin of their instability

The bond description for cluster compounds is commonly based on the molecular orbital diagrams, where the whole material is characterized by the specific electron population at various energetic levels. In contrast, this work is focused on the distribution of valence electrons between different liganding atoms, calculated first for a wide variety of octahedral metal cluster compounds (mainly Mo₆ and Re₆ chalcohalides) by the bond valence model. This distribution was found to be extremely nonuniform for most of the materials; it depends mostly on the ligand distances from the cluster center: The closer the ligand to the center, the higher is its charge (defined as its bond valence sum). To explain these results, a new model for the electrostatic interactions in the cluster compounds was proposed: The octahedral metal cluster with high polarizing power creates a strong electrostatic field, in which the adjacent separate anions behave as a part of a continuous dielectric medium, adjusting their charge to the distance from the cluster center. The valence violations related to this adjustment are the source of the material instability. Moreover, the bond valence analysis allows for the unveiling of the stabilization mechanisms existing in this type of cluster compounds. It was shown that the formation of mixed chalcohalides with an unusual anion arrangement, and chalcogen linkers with a reduced oxidation state, decreases the valence violation on the ligands. This work also illustrates additional ways of material stabilization, such as ion insertion inside the clusters or into the interstitials between the clusters, an increase in the number of inner ligands from 8 to 12, as well as increasing the connectivity of the structural units, Me₆X₈L₆ and Me ₆X₁₂L₆ (Me is the transition metal; X and L are internal and outer ligands, respectively).