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 CuIII complexes has intensified in recent years, focusing primarily on whether the [Cu(CF₃)₄]⁻ moiety should be described as a classical Werner-type 3d⁸ CuIII complex or as a 3d¹⁰ CuI inverted ligand field framework. The copper periodate complex [Cu(HIO₆)₂]⁵⁻, discovered in 1937, has long been regarded as a 3d⁸ CuIII species and sometimes used as a reference 3d⁸ CuIII complex in oxidation state assignments for Cu-containing metalloenzymes. Nevertheless, its detailed electronic structure remains unexplored. Herein, we revisit the oxidation state of [Cu(HIO₆)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(HIO₆)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 CuIII complexes as either 3d⁸ or 3d¹⁰ configurations is often inadequate; instead, the existence of electronic states at the boundary between these two limiting cases should be recognized.