Rational synthesis of unidimensional mixed valence solids. Structural, spectral, and electrical studies of charge distribution and transport in partially oxidized nickel and palladium bisdiphenylglyoximates

Abstract

A detailed study is presented of solid-state structure, oxidation state, and electron transport in Ni(dpg)/sub 2/I and Pg(dpg)/sub 2/I, dpg = diphenylglyoximato. The crystal structure of Ni(dpg)/sub 2/I has been studied by x-ray diffraction at 23 and -160/sup 0/C. The compound crystallizes in the tetragonal space group D/sub 4h//sup 8/-P4/ncc, with four formula units of Ni(dpg)/sub 2/I in a unit cell having dimensions at -160/sup 0/C of a = 19.774 (8), c = 6.446 (3) A. Full-matrix least-squares refinement of 31 variables gave a final value of the conventional R index (on F) of 0.093 for 353 reflections having F/sub 0//sup 2/ > 3 sigma(F/sub 0//sup 2/). The crystal structure consists of stacked Ni(dpg)/sub 2/ units (staggered by 90/sup 0/) and disordered chains of iodine atoms extending in the c direction. At -160/sup 0/C the Ni--Ni separation is 3.223 (2) A; other important distances are Ni--N = 1.868 (15), N--C = 1.33 (3), N--O = 1.34 (3) A. The Ni(dpg)/sub 2/ moiety is decidedly nonplanar (D/sub 2/ symmetry) with coordinated N atoms displaced 0.12 (2) A above or below the mean molecular plane. Analysis of diffuse x-ray scattering associated with the disordered iodine chains is consistent with I/sub 5//sup -/more » being the predominant species. This result is in agreement with resonance Raman spectroscopic studies on Ni(dpg)/sub 2/I and Pd(dpg)/sub 2/I as well as on model compounds. Fundamental polyiodide transitions in the M(dpg)/sub 2/I materials are observed(..nu../sub 0/ = 4880--6471 A)at 161 vs and 107 w cm/sup -1/; weak M(dpg)/sub 2/-centered scattering is also noted. Iodination brings about an increase in conductivity of > 10/sup 8/ (Ni) and > 10/sup 7/ (Pd), which is especially noteworthy since the only major change in crystal structure upon iodination is a ca. 0.27 A decrease in the M(dpg)/sub 2/ stacking distance. Variable-temperature studies show the Ni(dpg)/sub 2/I and Pd(dpg)/sub 2/I conductivities to be thermally activated.« less