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 ex periment with Cu-N distances within approximate to0.02-0.06 Angstrom. Howev er, 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. Howe ver, the electronic structure can be tuned to the experimental g values by modifying the copper nuclear charge. The 'd-d' transition energies were com puted from the optimised electronic state using Slater's transition state a pproach. DFT agrees with cellular ligand field (CLF) calculations and demon strates that amines are not pi -bonding ligands and that electrostatic inte ractions are not required for the CLF model. Instead, the observed spectros copic and structural data are successfully reinterpreted as arising from th e superposition of two, slightly different complexes in a ratio of approxim ately 5.5:1 and aligned at about 90 degrees to one another. This significan tly improves the agreement between the calculated and 'observed' structures . The implications for the interpretation of the 'd-d' spectra of other cop per(II) amine complexes are discussed.