Structural modifications of <i>M</i> ₅ O ₄ I ₁₁ ( <i>M</i> = Nb, Ta) cluster networks from heterogeneous solid-state reactions

The cluster compounds <i>M</i>₅O₄I₁₁ (<i>M</i> = 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 [<i>M</i>₅O₄] cluster core with metal (<i>M</i>) 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 <i>o</i>-Ta₅O₄I₁₁ and <i>m</i>-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.