Revisiting Formal Copper(III) Complexes: Bridging Perspectives with Quasi-d Configurations

The formal Cu(III) complex [Cu(CF₃)₄]^1- has often served as a paradigmatic example of challenging oxidation state assignment - with many reports proposing conflicting descriptions. Here we report a computational analysis of this compound, employing Energy Decomposition Analysis and Intrinsic Bond Orbital Analysis. We present a quasi-d perspective of the metal centre, resulting from ambiguities in d-electron counting. The implications for describing reactions which undergo oxidation state changes, such as the formal reductive elimination from the analogous [Cu(CF₃)₃(CH₂Ph)]^1- complex (Paeth et al. J. Am. Chem. Soc. 2019, 141, 3153), are probed. Electron flow analysis finds that the changes in electronic structure may be understood as a quasi-d to d transition at the metal centre, rendering this process essentially redox neutral. This is reminiscent of a previously studied formal Ni(IV) complex (Steen et al., Angew. Chem. Int. Ed. 2019, 58, 13133-13139), and indicates that our description of electronic structure has implications for the understanding of elementary organometallic reaction steps.