Pressure- and temperature-dependent isomerization reactions of aquabis(ethylenediamine) cobalt(III) complexes: deuterium solvent isotope effects

Abstract

The pressure and temperature dependence of the trans → cis isomerization reaction of trans-Co(en)₂(X)(OH₂)ⁿ⁺ (X = OH₂, OSeO₂H^- or AcO^-; en = ethane-1,2-diamine) has been determined in D₂O, and compared with results previously determined in H₂O under analogous conditions of acidity and ionic strength. Generally positive activation volumes (ΔV^‡) and activation entropies (ΔS^‡) in both solvents are consistent with a dissociative mechanism with aqua ligand release in the activated state. Isomerization in D₂O is characterized by slightly higher ΔH^‡ and ΔS^‡ values and a less positive ΔV^‡ value in each case. The clear change in ΔV^‡ from H₂O to D₂O supports the existence of an appreciable contribution from solvent rearrangement in forming the activated state to the activation volume. Variations in solvent structure and ionic solvation apparently account for the deuterium solvent isotope effect.