THE ELECTRONIC-STRUCTURE OF MIXED-VALENCE COPPER DIMER [CU-2(N(CH2CH2N=CHCH=NCH2CH2)(3)N)](3+)

Fully delocalised mixed-valence (MV) copper dimers, {Cu+1.5, Cu+1.5}, have been identified both in proteins and in model complexes. An inter pretation of the electronic transitions observed in the low-temperatur e magnetic circular dichroism and absorption spectra of one such dimer , [Cu2LimBT](3+), where L-imBT=N(CH2CH2N=CHCH=NCH2CH2)(3)N, is present ed. The spectrum is dominated by transitions within a set of energy le vels derived from almost pure copper 3d orbitals of the [Cd-2](3+) cor e. Limited ligand covalency is found. The MV transition energy has bee n used to elucidate the valence delocalisation energy which is compare d with the delocalisation energies in thiolate bridged MV copper dimer s. The delocalisation energy of 7100 cm(-1) is almost entirely due to copper-copper sigma overlap at a distance of 2.36 Angstrom and represe nts a metal-metal bond (sigma)(2)(sigma)(1) with bond order 0.5.