Positive volumes of activation and a dissociative mechanism for solvent exchange with [Mn(solvent)6]2+ ions

Abstract

Volumes of activation (ΔV^‡) determined by high-pressure ¹H n.m.r. spectroscopy for the exchange of the solvents N,N-dimethylthioformamide (dmtf) and N,N-dimethylformamide (dmf) with the corresponding [MX₆]²⁺(X = solvent) ions are, for X = dmtf, + 11.5 cm³ mol^–1(302 K) and +20.6 cm³ mol^–1(251 K) for M = Mn and Ni, respectively, and for X = dmf and M = Mn, +4.2 cm³ mol^–1(318 K); the significantly positive values of ΔV^‡ for Mn²⁺ contrast with the negative values obtained previously for this metal ion when X is water, methanol, or acetonitrile, indicating a mechanistic change from l_d to l_a for the less bulky solvent molecules.