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

TL;DR

This paper investigates Na4IrO4, revealing that weak Coulomb interactions allow Ir(IV) with a d^5 configuration to adopt a square-planar geometry, contrasting with typical d8/d9 complexes and highlighting the role of electron correlation.

Contribution

It demonstrates that weak on-site Coulomb interactions enable a d^5 transition metal to adopt a square-planar geometry, expanding understanding of electronic structure influences.

Findings

Na4IrO4 exhibits square-planar Ir(IV) coordination due to weak Coulomb repulsion.

Contrast with Na4CoO4 shows strong correlations lead to tetrahedral Co(IV) geometry.

Ab initio calculations support experimental observations of bonding and structure.

Abstract

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.