Structural modifications of M₅O₄I₁₁ (M = Nb, Ta) cluster networks from heterogeneous solid-state reactions

The cluster compounds M₅O₄I₁₁ (M = Nb, Ta) and Ta₅O₄I₁₁(TaI₅) were obtained from heterogeneous solid-state reactions and structurally characterised by single-crystal X-ray diffraction. Their crystal structures are based on the novel [M₅O₄] cluster core with metal (M) atoms arranged following the motif of a square pyramid. Iodide ligands contribute to different connectivities in the structures, resulting in (van der Waals type) waved layer structures. Two structural modifications exist for Ta₅O₄I₁₁, denoted as o-Ta₅O₄I₁₁ and m-Ta₅O₄I₁₁, and the compound Ta₅O₄I₁₁(TaI₅) encloses [TaI₅] molecules within voids of the structure. The two-dimensional nature of the structures and the presence of metal-to-metal bonding motivated investigations of the electronic properties through optical band-gap measurements, electrical conductivity studies, electronic band structure calculations, and X-ray photoelectron spectroscopy.