CHARGE-TRANSFER STATES OF BRIDGED TRANSITION-METAL DIMERS - MONOCLEAR VS BINUCLEAR COPPER AZIDE SYSTEMS WITH RELEVANCE TO OXY-HEMOCYANIN

The charge-transfer (CT) spectra of transition metal complexes show characteristic changes upon dimer formation. This study focuses on the (pi(nb)) --> Cu CT transitions of Cu azide systems which each split into two transitions if coordination to one copper center is replaced by bridging to two copper centers. In addition, a shift of the in-plane (pi(nb))sigma --> Cu CT transitions of the azide systems to lower energy is observed in the dimer spectrum. This shift is ascribed to a strong antiferromagnetic interaction in the CT excited state which is due to coupling of one electron in the bridging orbital with one unpaired electron on a copper center. Three models are developed and evaluated to interpret these CT excited-state shifts and splittings. The excitonic model evaluates only the diagonal energies (i.e. within the CT state) and gives a splitting scheme known from exciton theory which describes the dimer coupling in terms of four two-electron parameters. As the CT state splittings given by this model are too small, off-diagonal terms have to be considered. This is done in terms of molecular orbital (MO model) and valence-bond (VB) theory (VBCI model). It is shown that the VBCI model accounts for the sign and magnitude of the CT state splitting observed in Cu azide systems. In terms of this model, excited-state antiferromagnetism is described as configuration interaction (CI) with metal - metal CT (MMCT) and double CT (DCT) states. It is further shown that the VB and MO models agree in the description of triplet CT states and that the triplet CT state splitting corresponds to the HOMO-LUMO splitting of the dimeric complex. In order to obtain quantitative information regarding the CT transition energies and splittings, SCF-Xalpha SW calculations are performed on a structurally characterized Cu azide monomer and dimer. The implications of the excited-state interactions on the ground-state properties of bridged dimers are discussed.