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.
Keywords: ab initio calculations; coordination geometry; electronic structure; oxometalates; transition metals.
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