  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 d8 or d9 electron configuration. In
this work, we address an exotic square-planar mono-oxoanion [IrO4]^{4-} as
observed in Na4IrO4 with Ir(IV) in d^5 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
geometries.
