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

有机溶剂分子在非质子给体溶剂中与金属离子配位形成溶剂化团簇，溶剂分子之间同时与金属离子配位发生溶剂化立体效应。在溶液中络合反应的活化状态下，溶剂化立体效应也可能起作用。钴（II）离子在N， N-二甲基甲酰胺（DMF）和N， N-二甲基乙酰胺（DMA）中形成一系列卤代配合物。钴（II）离子在[CoCl_3（溶剂）]^-和[CoCl_4]^<2->+溶剂中呈四配位，根据反应速率的温度依赖性，确定了[CoCl_3（溶剂）]^- + Cl^- = [CoCl_4]^<2->+溶剂的活化焓和熵值。活化焓对反应速率起决定性作用，反应速率随活化焓的增大而减慢。结合基态热力学参数，得到了反应的激活焓。结果表明，在活化状态下溶剂化立体效应的作用更强，其表现形式与基态不同。在上述反应中，形成了五配位的[CoCl_4（溶剂）]^<2->。利用DMF向DMA的转移焓和在这些溶剂中的活化焓，估计[CoCl^4（溶剂）]^<2->从DMF向DMA的转移焓为26 kJ mol^<-1>，这表明Co^<2+>- o （DMA）键长比Co^<2+>- o （DMF）键长更长。另一方面，DMA的激活熵的下降比DMF小，表明DMA的运动自由度的下降比DMF小，这可能是由于Co^<2+>-O （DMA）的键长比Co^<2+>-O （DMF）的键长。

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.