Effect of an in-plane ligand on the electronic structures of bromo-bridged nano-wire ni-pd mixed-metal complexes, [Ni_1-xPd_x(bn) ₂Br]Br₂ (bn = 2S, 3S-diaminobutane)

Single crystals of quasi-one-dimensional bromo-bridged Ni-Pd mixed-metal complexes with 2S, 3S-diaminobutane (bn) as an in-plane ligand, [Ni _1-xPd_x(bn)₂Br]Br₂, were obtained by using an electrochemical oxidation method involving mixed methanol/2-propanol (1:1) solutions containing different ratios of [Ni^II(bn) ₂]Br₂ and [Pd^II(bn)₂]Br₂. To investigate the competition between the electron-correlation of the Ni ^III states, or Mott-Hubbard states (MH), and the electron-phonon interaction of the Pd^II-Pd^Iv mixed valence states, or charge-density-wave states (CDW), in the Ni-Pd mixed-metal compounds, X-ray structure analyses, X-ray oscillation photograph, and Raman, IR, ESR, and single-crystal reflectance spectra were analyzed. In addition, the local electronic structures of Ni-Pd mixed-metal single crystals were directly investigated by using scanning tunneling microscopy (STM) at room temperature and ambient pressure. The oxidation states of [Ni_1-xPd _x(bn)₂Br]Br₂ changed from a M ^II-M^Iv mixed valence state to a M^III MH state at a critical mixing ratio (x_c) of ∼0.8, which is lower than that of [Ni_1-xPd_x(chxn)₂Br]Br₂ (chxn=1 R,2R-diaminocyclohexane) (x_c≈ 0.9) reported previously. The lower value of x_c for [Ni_1-xPd_x(bn) ₂Br]Br₂ can be explained by the difference in their CDW dimensionalities because the three-dimensional CDW ordering in [Pd(bn) ₂Br]Br₂ observed by using X-ray diffuse scattering stabilizes the Pd^II-Pd^Iv mixed valence state more than two-dimensional CDW ordering in [Pd(chxn)₂Br]Br₂ does, which has been reported previously.