title: Na4IrO4 : square-planar coordination of a transition metal in d(5) configuration due to weak on-site coulomb interactions.
abstract: 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.
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, 01187 Dresden (Germany).
authors.forename: Sudipta
authors.initials: S
authors.lastname: Kanungo
authors.forename: Binghai
authors.initials: B
authors.lastname: Yan
authors.forename: Patrick
authors.initials: P
authors.lastname: Merz
authors.forename: Claudia
authors.initials: C
authors.lastname: Felser
authors.forename: Martin
authors.initials: M
authors.lastname: Jansen
keywords: ab initio calculations; coordination geometry; electronic structure; oxometalates; transition metals