This paper presents a detailed study of solid-state structure, oxidation state, and electron transport in Ni(dpg)2xI and Pd(dpg)2I, dpg = diphenylglyoximato. The crystal structure of Ni(dpg)2I has been studied by X-ray diffraction at 23 and -160 °C. The compound crystallizes from o-dichlorobenzene in the tetragonal space group D4h8-P4/ncc, with four formula units of Ni(dpg)2l in a unit cell having dimensions at -160 °C of a = 19.774 (8), c = 6.446 (3) Å. 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 F02 3σ(Fo2). The crystal structure consists of stacked Ni(dpg)2 units (staggered by 90°) and disordered chains of iodine atoms extending in the c direction. At -160 °C the Ni-Ni separation is 3.223 (2) Å other important distances are Ni-N = 1.868 (15), N-C = 1.33 (3), N-O = 1.34 (3) Å. The Ni(dpg)2 moiety is decidedly nonplanar (D2 symmetry) with coordinated N atoms displaced 0.12 (2) Å above or below the mean molecular plane. The form of iodine present, i.e., I2, I-, I3-, I5-, or mixtures thereof, cannot be determined from the Bragg scattering: analysis of diffuse X-ray scattering associated with the disordered iodine chains is consistent with 15- being the predominant species. This result is in agreement with resonance Raman spectroscopic studies on Ni(dpg)2I and Pd(dpg)2I as well as on an extensive series of selected model compounds. Fundamental polyiodide transitions in the M(dpg)2I materials are observed (vO = 4880-6471 Å) at 161 vs and 107 w cm-1; weak M(dpg)2-centered scattering is also noted. Iodine-129 Mössbauer data on Ni(dpg)2129] and Pd(dpg)2129] are also in best agreement with the 15- formulation. Averaging the data for both compounds, three sites with relative populations 2.04 (10):2.00 (10): 1.00 are observed with isomer shifts (vs. ZnTe) = 1.20 (3), 0.52 (5), 0.18 (1) mm/s and e2qQ = -1764 (5), -1331 (8), -880 (6) MHz, respectively. Thus, the M(dpg)2 units are formally in fractional oxidation states of +0.20 (4). Optical data show transitions at 566 (Ni(dpg)2I) and 505 nm (Pd(dpg)2I) which are most likely associated with the M(dpg)2 stacking interaction, and a broad band in both materials at 675 nm which is assigned to the polyiodide chains. X-ray photoelectron spectra (Ni2p3/2, Pd 3d3/2, 3d5/2) show no evidence of trapped valence. Single crystal electrical conductivity (both dc and 100-Hz ac) in the c (chain) direction is as high at 30 °C as 1.1 × 10>-1 (Ni(dpg)2I) and 4.7 × 10-3 (Ω cm)-1 (Pd(dpg)2I). Iodination brings about an increase in conductivity of > 108 (Ni) and > 107 (Pd), which is especially noteworthy since the only major change in crystal structure upon iodination is a ca. 0.27 Å decrease in the M(dpg)2 stacking distance. Variable-temperature studies show the Ni(dpg)2 [and Pd(dpg)2l conductivities to be thermally activated, with activation energies of 0.19 ± 0.01 and 0.54 ± 0.11 eV, respectively.
ASJC Scopus subject areas
- Colloid and Surface Chemistry