Kinetic solvation steric effect at the transition state of reaction between trichlorocobaltate(II) and chloride ions in N,N-dimethylformamide and dimethyl sulfoxide

The rate of the solvent substitution reaction, CoCl3S−+Cl−→CoCl42−+S (S-solvent) has been studied by means of stopped flow spectrophotometry in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF), and the rate constant kf, and the activation enthalpy ΔfH≠ and entropy ΔfS≠ have been determined. The negative and large ΔfS≠ values of −38 and −68 J K−1 mol−1 in DMSO and DMF, respectively, evidently indicate that an associative mechanism operates or the five-coordinate species CoCl4S2− forms at the transition state. The corresponding ΔfH≠ values are 48 and 36 kJ mol−1 in DMSO and DMF, respectively. To discuss solvation enthalpy difference of the CoCl4S2− at the transition state in the solvents, the transfer enthalpy ΔtH≠ of the activated species from DMF to DMSO was evaluated by combining the ΔfH≠ values and the enthalpies of transfer ΔtH° of CoCl3− and Cl− from DMF to DMSO. The ΔtH≠ value thus obtained is 18 kJ mol−1, the value indicating that the solvation enthalpy of the activated species in DMF is more negative than that in DMSO despite that the electron-pair donating ability of DMF is weaker than that of DMSO. This implies that the entering chloride ion suffers from an appreciable steric hindrance of DMSO at the transition state.