MX-Chain Compounds with ReO₄ Counterions: Exploration of the Robin–Day Class I–II Boundary

MX chains have been widely studied as a 1D mixed-valence system. Although there have been a large number of studies on the boundary between class II and III materials of the Robin–Day classification, there are few studies of compounds at the boundary between classes I and II. In this study, we synthesized a series of Pt- and Pd- MX-chain compounds with perrhenate counterions, [M­(en)₂]­[M­(en)₂X₂]­(ReO₄)₄ (X = Br for M = Pd and X = Cl, Br, and I for M = Pt). All compounds were isostructural, and the metal–metal distances within the chain exceed 6 Å, which is the longest among MX-chain compounds thus far reported. For [Pt­(en)₂]­[Pt­(en)₂Cl₂]­(ReO₄)₄ (PtCl), an intervalence charge transfer (IVCT) transition was observed in the UV region at 335 nm (3.7 eV), which is the shortest wavelength for the MX-chain compounds thus far reported, indicating that PtCl is the closest to the Robin–Day class I limit.