SYNTHESIS, CRYSTAL-STRUCTURE, ELECTRICAL-PROPERTIES, AND ELECTRONIC BAND-STRUCTURE OF (NHYME4-Y)X[M(DMIT)2] COMPLEXES (M=NI, PD, PT DMIT(2-) = 2-THIOXO-1,3-DITHIOLE-4,5-DITHIOLATO)

Seven phases of the family (NHyMe4-y)x[M(dmit)2] (M = Ni, Pd, Pt) have been isolated as single crystals by the electrocrystallization technique: (NMe4)0.5[Ni(dmit)2] (1), (NHMe3)0.5[Ni(dmit)2] (2), (NHMe3)2[Ni(dmit)2]5.2MeCN (3), (NH2Me2)0.5[Ni(dMit)2] (4), (NH3Me)2[Ni(dmit)2]5.2MeCN (5), (NH4)x[Ni(dmit)2] (6), and (NHMe3)([Pt(dmit)2]3.MeCN(7). All these phases are built of stacked M(dmit)2 entities forming anionic slabs which alternate with sheets containing the cations (and the solvent molecules when present). In phases 1, 2, and 4 the Ni(dmit)2 entities are paired and form weak dimers. Phases 1 and 2 are highly conductive: sigma(RT) = 55 and 140 S.cm-1, respectively. At ambient pressure a metal-like temperature-dependent behavior is observed down to 90 and 220 K respectively. Under pressure the superconducting transition reported previously for 1 is not observed; nonohmic behavior is observed at low temperature. Comparison of the crystal and electronic band structures of 1 and 2 indicates that the nature of the pseudometal-to-insulator transition of 1 cannot result from a CDW instability while such an instability cannot be ruled out in 2. Phase 4 is semiconductive (sigma(RT) = 0.1 S.cm-1; E(a) = 0.21 eV). Phases 3 and 5 are also semiconductive (sigma(RT) = 0.2 and 1.5 S.cm-1; E(a) = 0.23 and 0.20 eV). In these phases with 2:5 stoichiometry, the repeating unit of a stack is made of two weak dimers and one single molecule; tight-binding band structure calculations suggest that the two electrons are localized on the two weak dimers while the single molecule is neutral. All samples of 6 are found to be semiconductive (sigma(RT) = 0.2-0.7 S.cm-1 in agreement with previous work); the metallic phase of 6 described by other authors is not found. The conductivity of phase 7 (sigma(RT) = 140 S.cm-1) is the highest observed in Pt(dmit)2 compounds; it remains almost constant down to 180 K and then drops sharply. The crystal structure of 7 is made of stacks of [Pt(dmit)2]2 dimers characterized by strong Pt-Pt interactions. In contrast to 1 and 2 where intrastack interactions are predominant, band structure calculations for 7 indicate the dominant character of the interstack interactions in the conduction process.