Na4IrO4: Singular manifestation of square-planar coordination of a transition metal in d configuration due to weak on-site Coulomb interaction

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition metal complexes. For example, square-planar surroundings found in transition oxometalates such as curprates are usually associated with the d or d electron configuration. In this work, we address an exotic square-planar mono-oxoanion [IrO4] as observed in Na4IrO4 with Ir(IV) in d configuration, and characterize the chemical bonding by experiment and ab initio calculations. We find that Na4IrO4 in its ground state evolves a square-planar coordination for Ir(IV) because of the weak Coulomb repulsion of Ir-5d electrons. In contrast, in its 3d counterpart, Na4CoO4, Co(IV) is in tetrahedral coordination, due to strong electron correlation. Na4IrO4 thus may serve as a simple paradigmatic platform for studying the ramifications of Hubbard type Coulomb interactions on local