This paper presents a detailed study of crystal structure, stoichiometry, oxidation state, and electron transport in the materials Ni(bqd)2, Pd(bqd)2, Ni(bqd)2I0.018, Ni(bqd)2l0.5.S, and Pd(bqd)2I0.5.S, where bqd = o-benzoquinonedioximato and S = an aromatic solvent. The compound Pd(bqd)2l0.50.0.52o-C6H4C12 has been shown by single-crystal X-ray diffraction to crystallize in the tetragonal space group D4h 2-P4/mcc, with four formula units in a cell of dimensions a = 16.048 (7) and c = 6.367 (3) Å. Full-matrix least-squares refinement gave a final value of the conventional R index (on F) of 0.052 for 1278 reflections having Fo 2 > 3σ(Fo 2). The crystal structure consists of stacked Pd(bqd)2 units, each staggered by 65° with respect to its nearest neighbors, and disordered chains of iodine atoms extending in the c direction. The solvent molecules are disordered throughout tunnels which extend parallel to c. The Pd-Pd distance is 3.184 (3) Å, Pd-N = 1.996 (7) Å, and the Pd(bqd)2 units are rigorously planar. Resonance Raman studies (vo 4880-6471 Å) of Pd(bqd)2I0.5.S and Ni(bqd)2I0.5.S indicate that the predominant form of the iodine present is I3 - (Vfundamental 107 cm-1), hence that the formal charge on the M(bqd)2 units is +0.17 (2). Iodine-129 Mössbauer studies are also consistent with the I3 - formulation. Optical spectra of these complexes exhibit a strong, broad transition at 600 nm which is largely, if not exclusively, due to the polyiodide chains. Crystallization of Ni(bqd)2 from benzene containing traces of iodine produces the new orthorhombic phase Ni(bqd)2I0.018. Singlecrystal X-ray studies have shown it to crystallize in the space group D26 2h-Ibam with four formula units in a unit cell of dimensions a = 16.438 (2), b = 14.759 (4), and c = 6.360 (2) Å. Full-matrix least-squares refinement gave a final value of the conventional R index (on F) of 0.11 for 1026 reflections having Fo 2 > 3σ(Fo 2). The structure contains rigorously planar Ni(bqd)2 units stacked along the c axis with each molecule staggered by 68° with respect to its nearest neighbors. The Ni-Ni distance is 3.180 (2) Å and Ni-N = 1.88 (10) Å. Structural relationships are discussed for all M(bqd)2 and M(bqd)2IA compounds; partial oxidation results in a small contraction of the interplanar spacings of 0.019 Å (Pd) to 0.027 Å (Ni). Single-crystal electrical conductivity measurements (dc and 100-Flz ac) in the stacking direction show an increase in conductivity upon partial oxidation of >103 (Ni) and > 104 (Pd). Maximum conductivities at 300 K are Ni(bqd)2I0.52.0.32C6H5CH3 = 1.1 × 10-5 (Ω cm)-1 and Pd(bqd)2l0.5.0.52o-C6H4Cl2 = 8.1 × 10-3 (Ω cm)-1. Variable-temperature studies show that the electrical conductivity follows, over the entire range investigated, an exponential temperature dependence with a single activation energy: 0.54 ± 0.08 (Ni) and 0.22 ± 0.03 eV (Pd).
ASJC Scopus subject areas
- Colloid and Surface Chemistry