Pressure-induced non-innocence in bis(1,2-dionedioximato)Pt(ii) complexes: an experimental and theoretical study of their insulator-metal transitions.

Bis(1,2-dionedioximato) complexes of Pt(ii) are known for their propensity to form linear chains of metal complexes in the solid state, and under the application of pressure members of the family display interesting optical and conductive properties. Two examples, Pt(bqd) and Pt(dmg), are known to undergo insulator-to-metal-to-insulator transitions, with the metallic state reached at 0.8-1.4 GPa and 5 GPa, respectively. Previous interpretations of these materials' behaviour focused on the role of the filled d and vacant p orbitals on platinum, with little consideration to the role of the ligand. Here, the pressure-structural behaviour of Pt(bqd) is investigated through single crystal X-ray diffraction, the first such study on this material. The difference in conductive behaviour under pressure between Pt(bqd) and Pt(dmg) is then interpreted through a combination of experimental and computational methods, including conductivity measurements under high pressure and electronic structure calculations. Our computational work reveals the significant contribution from ligand low-lying vacant π-orbitals to the frontier orbitals and bands in these complexes, and provides an explanation for the experimentally observed re-entrant insulator-to-metal-to-insulator transitions, and the differences in behaviour between the two compounds.