HALOGEN-BRIDGED ONE-DIMENSIONAL NI-II-NI-IV MIXED-VALENCE STATE AND NI-III MOTT-HUBBARD STATE

The halogen-bridged nickel compounds with linear-chain structure conta ining hetero-halogens as bridging and counter anions, Ni(chxn)2Cl2 459 Br0.541 (2) and Ni(chxn)2Cl1.28Br1.72 (3) (chxn=1R, 2R-diaminocyclohex ane), have been synthesized by bromination of Ni(chxn)2Cl2 and chlorin ation of Ni(chxn)2Br2 in 2-methoxyethanol, respectively. Their structu res. have been determined by single-crystal X-ray diffraction method. In both (2) and (3), the Ni(chxn)2 moieties are bridged by halogens to construct linear-chain structures and hydrogen-bond networks of NH... X...HN which hold the chains are extended over the chains two-dimensio nally. The bridging halogens in (2) and (3) are located at the midpoin ts between two nickel atoms on the chains, that is, -N(III)-X-Ni(III)- structure with no Pelerls distortion, indicating that these compounds take the Mott-Hubbard state as the same as the compounds containing ho mo-halogens, Ni(chxn)2Cl3 (1) and Ni(chxn)2Br3 (4) reported previously . The occupancy factors of Cl and Br in bridging and counter halogen s ites in (2) and (3) are determined by X-ray analyses: [Ni(chxn)2ClxBr1 -x]ClyBr2-y, x=0.976 and y=1.483 in (2) and x=0.68 and y=0.60 in (3). They show intense charge transfer bands in the visible region only wit h an electric vector parallel to the 1-D chain and their energies are in the order, (1)>(2)>(3)>(4). Substitution of the counter Cl-ions in [Ni(chxn)2Cl]Cl2 (1) With more bulky ClO4- ions succeeded in producing Ni(chxn)2CI(ClO4)2 by a heterogeneous reaction. The solid state C-13 NMR spectra suggest that Ni(chxn)2Cl(ClO4)2 take not the Mott-Hubbard state but the Ni(II)-Ni(IV) mixed-valence state formulated as [Ni(chxn )2][NiCl2(chxn)2](ClO4)4. These results indicate that the oxidation st ates of Ni atoms in these comopounds are strongly affected by the kind of counterion through the hydrogen-bond and/or bulkiness.