Structure of solvent cluster and solvation steric effect at the transition state

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Organic solvent molecules coordinate to the metal ion to form solvation clusters in an aprotic donor solvent, and solvation steric effect operates between solvent molecules simultaneously coordinating to the metal ion. Solvation steric effect may also operate at the activated state of a complexation reaction in solution. The cobalt (II) ion forms a series of halogeno complexes in N, N-dimethylformamide (DMF) and N, N-dimethylacetamide (DMA). The cobalt (II) ion is four-coordinated in both [CoCl_3 (solvent)]^- and [CoCl_4]^<2->, and activation enthalpy and entropy values for reaction, [CoCl_3 (solvent)]^- + Cl^- = [CoCl_4]^<2->+ solvent, have been determined from temperature dependence of the reaction rate. The activation enthalpy play a decisive role in the reaction rate, and the rate become slower with increasing enthalpy of activation. By combining thermodynamic parameters at the ground state, the activation enthalpy for the reverse reaction was also obtained. It is concluded that solvation steric effect operates more strongly at the activated state, and its aspect is different from that at the ground state. In the above reaction, the five-coordinated [CoCl_4 (solvent)]^<2-> is formed. By using enthalpies of transfer of related species from DMF to DMA and those of activation in these solvents, the enthalpy of transfer of [CoCl^4 (solvent)]^<2-> from DMF to DMA was estimated to be 26 kJ mol^<-1>, and this indicating that the Co^<2+>-O (DMA) bond length become longer than the Co^<2+>-O (DMF) bond length. On the other hand, the decrease in the entropy of activation is smaller for DMA than DMF, indicating that the decrease in freedom of motion is smaller for DMA than DMF, probably owing to longer Co^<2+>-O (DMA) bond length than the Co^<2+>-O (DMF) bond length.