Revisiting the Oxidation State of [Cu(HIO ₆ ) ₂ ] ^5– : A Copper(III) Complex between Classical Werner-Type and Inverted Ligand Fields

Debate over the electronic structure of Cu III complexes has intensified in recent years, focusing primarily on whether the [Cu(CF 3 ) 4 ] − moiety should be described as a classical Werner-type 3d 8 Cu III complex or as a 3d 10 Cu I inverted ligand field framework. The copper periodate complex [Cu(HIO 6 ) 2 ] 5–, discovered in 1937, has long been regarded as a 3d 8 Cu III species and sometimes used as a reference 3d 8 Cu III complex in oxidation state assignments for Cu-containing metalloenzymes. Nevertheless, its detailed electronic structure remains unexplored. Herein, we revisit the oxidation state of [Cu(HIO 6 ) 2 ] 5– by means of X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and density functional theory calculations. The obtained results show that the oxidation state of the Cu center in [Cu(HIO 6 ) 2 ] 5– lies at the boundary between the classical Werner-type and inverted ligand field regimes. This study thus demonstrates that categorizing the oxidation state of Cu III complexes as either 3d 8 or 3d 10 configurations is often inadequate; instead, the existence of electronic states at the boundary between these two limiting cases should be recognized.