Tuning of Spin Density Wave Strengths in Quasi-One-Dimensional Halogen-Bridged Ni^III Complexes with Strong Electron Correlations, [Ni^III(chxn)₂X]Y₂

A series of quasi-one-dimensional halogen-bridged Ni^III complexes, [Ni(chxn)₂X]Y₂ (chxn = 1R,2R-diaminocyclohexane; X = Cl, Br, and mixed halides; Y = Cl, Br, mixed halides, NO₃, BF₄, and ClO₄) have been synthesized in order to investigate the effect of the bridging halogens and counteranions on their crystal, electronic structures, and moreover the spin density wave strengths. In the crystal structures, the [Ni(chxn)₂] moieties are symmetrically bridged by halogen ions, forming linear-chain Ni^III−X−Ni^III structures. The hydrogen bonds between the aminohydrogens of chxn and the counteranions are constructed not only along the chains but also over the chains, forming the two-dimensional hydrogen-bond networks. While the Ni^III−X−Ni^III distances or b axes are almost constant in the compounds with the same bridging halogens, the c axes which correspond to the interchain distances in the directions of the interchain hydrogen bonds are remarkably lengthened with the increase of the ionic radius of the counterions; X < NO₃ < BF₄ < ClO₄. These compounds show the very strong antiferromagnetic interactions among spins on Ni 3d_z² orbitals through the superexchange mechanisms via the bridging halogen ions. Judging from the results of X-ray photoelectron spectra (XPS), Auger spectra, and single-crystal reflectance spectra, these Ni compounds are not Mott-insulators but charge-transfer-insulators. Their electronic structures or the spin density wave strengths are found to be tuned by the combinations of the counteranions and the bridging halogens.