Observation of an Alternate Charge‐Polarization State in a One‐Dimensional Pt−Pt−I Chain Compound with a Bulky Pendant Ligand

A one‐dimensional (1D) halogen‐bridged dinuclear‐metal complex (MMX‐chain) exhibits various electronic states based on a mixed‐valence metal‐dimer system. This report deals with the synthesis and physical properties of a new MMX‐chain with a bulky pendant ligand, Pt2(mcc‐HexCS2)4I (mcc‐HexCS2=trans‐4‐(methoxycarbonyl)cyclohexanedithiocarboxylate). The steric hindrance caused by the bulky substituent induces a strain in its 1D chain, achieving at ambient condition the first pure alternate charge‐polarization (ACP) state (−Pt2+−Pt3+−I−−Pt3+−Pt2+−I−−), a kind of spin‐Peierls state, as confirmed by X‐ray diffraction and its conducting and magnetic properties. The strain effect is also manifested as a very large linear thermal expansion along the chain direction, which is quite different from conventional MMX‐chains without a bulky ligand. The design of low‐dimensional materials with ligand variations is expected to lead to the emergence of new electron–lattice coupled electronic states.