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 interpretation of the electronic transitions observed in the low-temperature magnetic circular dichroism and absorption spectra of one such dimer, [Cu2LimBT](3+), where L-imBT=N(CH2CH2N=CHCH=NCH2CH2)(3)N, is presented. The spectrum is dominated by transitions within a set of energy levels derived from almost pure copper 3d orbitals of the [Cd-2](3+) core. Limited ligand covalency is found. The MV transition energy has been used to elucidate the valence delocalisation energy which is compared with the delocalisation energies in thiolate bridged MV copper dimers. The delocalisation energy of 7100 cm(-1) is almost entirely due to copper-copper sigma overlap at a distance of 2.36 Angstrom and represents a metal-metal bond (sigma)(2)(sigma*)(1) with bond order 0.5.