Thermochemistry of cis trans isomerization in MCl2L2(PPh3)2, M=Ru, Os; L=CO, CNCH2SO2-p-tolyl.

Ten neutral dichlorobistriphenylphosphine complexes of ruthenium(II) and osmium(II) with two π-accepting ligands, CO or CNCH₂SO₂-p-tolyl (TosMIC), have been prepared and characterized. There are five pairs of complexes, each with trans-triphenylphosphines and either cis or trans arrangement of the two chlorides and two π-accepting ligands. The all-trans arrangement, ttt-MCl₂L₂(PPh₃)₂ are prepared under kinetic control by adding 2 L to MCl₂(PPh₃)₃. In most cases these can be stoichiometrically thermally isomerized to the more thermodynamically stable cct-MCl₂L₂(PPh₃)₂ as either a solid or in solution with high boiling solvent. Mixed carbonyl/isocyanide complexes form by treating ttt-MCl₂(CO)₂(PPh₃)₂ with excess isocyanide at room temperature to give ttt-MCl₂(CO)(CNR)(PPh₃)₂ with retention of stereochemistry at the metal. Thermolysis of these mixed species also cleanly transform to the thermodynamically stable cct-isomers. Differential scanning calorimetry is used to determine the temperature onset (155–229 °C) and enthalpy (19–92 J/mol) for these isomerizations. In examples with at least one CO ligand, these DSC methods allow for Borchardt-Daniels kinetics determination of the energy of activations for these isomerizations between 169–282 kJ/mol. Single crystal X-ray diffraction confirms the spectroscopically assigned geometries and DFT calculations of the most likely intermediates for carbonyl substitution in ttt-MCl₂(CO)₂(PPh₃)₂ supports carbonyl loss as the initial step in these substitution reactions. It is proposed that isomerization competes with geminate recombination of the dissociated CO or CNR to give the starting isomer ttt-MCl₂L₂(PPh₃)₂.