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 ⁴B_2u ground state.