Paramagnetism at ambient temperature, diamagnetism at low temperature in a Ru₂⁶⁺ core: Structural evidence for zero-field splitting

Variable temperature magnetic studies of the Ru₂⁶⁺ guanidinate compounds Ru₂(hpp)₄Cl₂ (1) and Ru₂(hpp)₄(CF₃-SO₃)₂ (2) show that they are paramagnetic with two unpaired electrons at room temperature and that they appear essentially diamagnetic at 2 K. In neither compound do the Ru-Ru distances vary by more than 0.008(1) Å from 27 to 296 K. This argues strongly that the ground state electronic configuration remains constant over this temperature range and that the decrease in magnetism as the temperature is lowered must be attributable to zero-field splitting of the ³A _2g ground state arising from the electronic configuration σ²π⁴δ²π*². The Ru-Ru distance in 1 is about 0.04-0.05 A longer than that in 2 which indicates that the Ru₂(hpp)₄²⁺ core is quite sensitive to the nature of the axial ligands. The electronic spectra show three absorption bands for each compound.