Electronic structure of the quasi-one-dimensional halogen-bridged Ni complexes [Ni(chxn)(2)X]X(z) (X=Cl, Br) and related Ni compounds

The electronic structure of the one-dimensional Ni complexes, [Ni(chxn)(2)X]X(2)(X=Cl, Br; (chxn)=1R,2R-cyclohexanediamine), is studied together with the discrete Ni complexes, [NiX(2)([14]aneN(4))]ClO4(X = Cl; Br;([14]aneN(4))=1,4,8,11-tetraazacyclotetradecane), using optical spectroscopy, x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. The optical spectra show that the Br compounds have a smaller gap as compared with the Cl compounds. An analysis using a simple X-Ni-X trimer model on the optical spectra and the Ni 2p XP spectra yields quantitative estimates for the charge transfer (CT) energy Delta and the transfer energy T for discrete and one-dimensional Ni complexes. The analysis on the Ni LVV Auger spectra in conjunction with the valence XP spectra indicates that the average on-side d-d Coulomb energy U in the one-dimensional and discrete Ni complexes is about 5 eV, quite similar to the case of the Ni dihalides. The obtained results demonstrate that the one-dimensional Ni complexes are CT insulators. We discuss the differences in the electronic structures of the one-dimensional Ni complexes compared with the Ni dihalides and the one-dimensional Pt complexes on the basis of the estimated parameter values of Delta, T, and Li.