Electronic structure of the quasi-one-dimensional halogen-bridged Ni complexes [Ni(chxn)2X]X2(X=Cl, Br) and related Ni compounds

The electronic structure of the one-dimensional Ni complexes, [Ni(chxn${)}_2$X]${\mathit{X}}_2$(X=Cl, Br; (chxn)=1R,2R-cyclohexanediamine), is studied together with the discrete Ni complexes, [Ni${\mathit{X}}_2$([14]${\mathrm{aneN}}_4$)]${\mathrm{ClO}}_4$(X=Cl; Br;([14]${\mathrm{aneN}}_4$)=1,4,8,11-tetraazacyclote tradecane),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 Δ 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 Δ, T, and U. © 1996 The American Physical Society.