The [(DT-TTF)₂M(mnt)₂] family of radical ion salts: From a spin ladder to delocalised conduction electrons that interact with localised magnetic moments

Electrocrystallisation of the π-electron donor dithiopheno-tetrathiafulvalene (DT-TTF) with maleonitrile dithiolate (mnt)-metal (M) complexes gives rise to the new family of radical ion salts [(DT-TTF)₂M(mnt)₂] (M = Au, Pt, Ni), which are isostructural and crystallise in the monoclinic space group P2₁/n forming regular segregated stacks of donor and acceptor molecules along the b axis. The DT-TTF stacks are paired and interact strongly through S ⋯ S contacts in a ladder-like motif. The three salts have quite high room-temperature electrical conductivities (9, 40 and 40 S cm^-1 for M = Au, Pt and Ni respectively) but their conductivity-temperature dependencies differ. The Au salt has an activated conductivity at room temperature whereas the Ni and Pt salts are metal-like at room temperature and both exhibit a metal-insulator transition around 120 K. These contrasting transport properties are accounted for by the differences in the transfer integrals along the DT-TTF stacks. The magnetic susceptibility of the salt with M = Au, in which the [Au(mnt)₂^-] anion is diamagnetic, can be fitted to a two-legged spin-ladder model. From diffuse X-ray scattering studies it is established that below 220 K the donors dimerise along the b stacking direction, and the spin carrier units in the ladder are identified as those formed by dimerised donors [(DT-TTF)₂]^+·. Observation in their EPR spectra of a single line which increases dramatically in width as the conductivity increases is evidence for the presence of two magnetic subsystems which interact in the salts (M = Ni, Pt) with paramagnetic [M(mnt)₂]^- ions.