A systematic K-edge X-ray absorption spectroscopic study of Cu(III) sites

Highly oxidized metals are constituents of oxidants, reactive intermediates, and materials with interesting conductive and magnetic properties. High-energy spectroscopies have played an important role in identifying and describing the bonding character of highly oxidized metals in these materials. A systematic study of Cu(III) K-edge X-ray absorption spectra was carried out to identify analytically useful signatures of Cu(III) in the K-edge, and to elucidate bonding descriptions for Cu(III)-containing complexes. K-edges for six Cu(III) complexes and their same-ligand Cu(II) counterparts are compared. Edges for the Cu(III) species generally appear at higher energies than their Cu(II) counterparts, though energy shifts between most individual edge features vary. However, for all Cu(III) compounds studied, the 1s --> 3d transition in the preedge energy range exhibits a distinct, 2 eV shift to higher energy, relative to the known and relatively unvarying energy of the 1s --> 3d transition in Cu(II) species. This energy shift provides a direct means of distinguishing Cu(III) from Cu(II). The K-edge for a complex containing Cu(II) coordinated to a 1e(-)-oxidized ligand (phenoxyl) does not show such a change in the 1s --> 3d transition energy. The analytical, potential of the Cu K-edge was tested with good success using a mixed-valent trinuclear species. Cu(III) is detectable using the K-edge. The limitations of the K-edge as a Cu(III) analytical probe are discussed. An analysis applied to the 1s --> 4p and 1s --> 4p + shakedown transitions in the edge for a {Cu-2(II)(mu-OH)(2)}(2)(+) dimer, using a configurational interaction (CI) model, predicted similar to 75% d-character in the ground state. A similar analysis of the K-edge for (Cu-2(III)(mu-O)(2)}(2+) indicates that the Cu in this complex has far more covalent bonds with the oxo bridging ligands (d-character similar to 60%).