In spite of a long history of quasi-one-dimensional halogen-bridged complexes (MX-chains), all Pt and Pd complexes form charge-density-wave (CDW) state (-X⋯M 2+⋯XM 4+-X⋯), while all Ni complexes form Mott-Hubbard (MH) state (-XNi 3+-XNi 3+-X-), without exception. Although an interesting charge dynamics are expected at the boundary between the two states, no compound thus far reported forms both states. Thus, we have sought a phase boundary in bromo-bridged Pd compounds by decreasing Pd⋯Pd distance. In this article, we introduce the following two methods to decrease Pd⋯Pd distances: introduction of long alkyl chains which can afford attractive force between them, and partial substitution with smaller Ni ions, [Ni 1-xPd x(chxn) 2Br] Br 2. In both systems, we have succeeded in realizing Pd 3+ MH state. It has been revealed that the electronic state of bromo-bridged Pd compounds is determined by the Pd⋯Pd distances, in other words, CDW and MH states are stabilized when Pd⋯Pd distances are longer and shorter than 5.26 Å, respectively.
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