Na₄IrO₄: Square-Planar Coordination of a Transition Metal in d⁵ Configuration due to Weak On-Site Coulomb Interactions

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition-metal complexes. For example, square-planar coordination geometries found in transition-metal oxometalates such as cuprates are usually associated with the d⁸ or d⁹ electron configuration. In this work, we address an unusual square-planar single oxoanionic [IrO₄]^4- species, as observed in Na₄IrO₄ in which Ir^IV has a d⁵ configuration, and characterize the chemical bonding through experiments and by ab initio calculations. We find that the Ir^IV center in ground-state Na₄IrO₄ has square-planar coordination geometry because of the weak Coulomb repulsion of the Ir-5d electrons. In contrast, in its 3d counterpart Na₄CoO₄, the Co^IV center is tetrahedrally coordinated because of strong electron correlation. Na₄IrO₄ may thus serve as a simple yet important example to study the ramifications of Hubbard-type Coulomb interactions on local geometries.