Exploration of boundary between charge-density-wave and Mott-Hubbard states in quasi-one-dimensional halogen-bridged metal complexes

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.