Na4IrO4 : square-planar coordination of a transition metal in d(5) configuration due to weak on-site coulomb...

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 d(8)  or d(9)  electron configuration. In this work, we address an unusual square-planar single oxoanionic [IrO4 ](4-)  species, as observed in Na4 IrO4 in which Ir(IV) has a d(5)  configuration, and characterize the chemical bonding through experiments and by ab initio calculations. We find that the Ir(IV)  center in ground-state Na4 IrO4 has square-planar coordination geometry because of the weak Coulomb repulsion of the Ir-5d electrons. In contrast, in its 3d counterpart Na4 CoO4 , the Co(IV) center is tetrahedrally coordinated because of strong electron correlation. Na4 IrO4 may thus serve as a simple yet important example to study the ramifications of Hubbard-type Coulomb interactions on local geometries.