A combined ligand field and density functional theory study of the structural and spectroscopic properties of [Cu(dien)(2)](2+)

Issues regarding assignment of the 'd-d' spectrum of [Cu(dien)(2)]Br-2.H2O have been resolved using density functional theory (DFT) calculations. The fully optimised structure of [Cu(dien)(2)](2+) is in good agreement with experiment with Cu-N distances within approximate to0.02-0.06 Angstrom. However, one axial contact is 0.22 Angstrom longer than reported and the ground state Cu-N covalency is overestimated. This leads to computed EPR g values which are too low and to 'd-d' transition energies which are too high. However, the electronic structure can be tuned to the experimental g values by modifying the copper nuclear charge. The 'd-d' transition energies were computed from the optimised electronic state using Slater's transition state approach. DFT agrees with cellular ligand field (CLF) calculations and demonstrates that amines are not pi -bonding ligands and that electrostatic interactions are not required for the CLF model. Instead, the observed spectroscopic and structural data are successfully reinterpreted as arising from the superposition of two, slightly different complexes in a ratio of approximately 5.5:1 and aligned at about 90 degrees to one another. This significantly improves the agreement between the calculated and 'observed' structures. The implications for the interpretation of the 'd-d' spectra of other copper(II) amine complexes are discussed.