text: Sigma non-innocence has been extensively discussed for the prototypical example of a copper complex [Cu(CF)] in conjunction with the concept of an inverted ligand field. In 1995, Snyder suggested, based on his quantum chemical calculations, that this formal Cu(III) (d) complex would be more appropriately represented as a Cu(I) (d) complex. Snyder pointed out that the frontier molecular orbitals of [Cu(CF)] are dominated by ligand parentage due to the higher-lying ligand orbitals compared to the metal orbitals, and this inversion of the ligand field causes the d orbital to be occupied and the lowest unoccupied molecular orbital (LUMO) to be ligand centered.
  Later, Lancaster et al. experimentally validated this inverted ligand field electronic structure of [Cu(CF)] using core spectroscopy techniques. Their findings revealed that the 3d orbitals are nearly fully occupied, supporting the formulation of this ion as a Cu(I) species. The assignment of what would be typically called a Cu(III) species as Cu(I) indicates the sigma non-innocence of the perfluoromethyl ligands in the complex.
  The researchers also examined the electronic structure of other formally Cu(III) complexes using Cu L-edge X-ray absorption spectroscopy together with computational techniques. They reported that all the Cu(III) species they studied except CuF have significantly diminished metal d-character in their LUMOs compared to the formal d assignment. This implies that ligand field inversion and sigma non-innocence are not unique to [Cu(CF)] but is general in many systems.