Structural and Magnetic Evidence Concerning Spin Crossover in Formamidinate Compounds with Ru₂⁵⁺ Cores

The magnetic and structural properties of two Ru2(DArF)4Cl compounds, where DArF is the anion of a diaryl formamidine, are presented here. The compounds with Ar = p-anisyl and m-anisyl both show temperature dependence of χT (χ = molar magnetic susceptibility), but for different reasons. For the para compound, there is a Boltzmann distribution between a π*3 ground state and a δ*π*2 upper state, and this is confirmed by a temperature dependence of the Ru−Ru bond length: 2.4471(5) Å at 23 K and 2.3968(5) Å at 300 K. For the meta compound, a δ*π*2 configuration persists over the range of 23−300 K as shown by an invariant Ru−Ru bond length, but the χT drops with decreasing temperature owing to zero-field splitting of a 4B2u ground state.