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

在非质子供体溶剂中，有机溶剂分子与金属离子配位形成溶剂化簇，同时与金属离子配位的溶剂分子之间发生溶剂化空间效应。溶剂化空间效应也可以在溶液中络合反应的活化状态下起作用。钴（II）离子在N，N-二甲基甲酰胺（DMF）和N，N-二甲基乙酰胺（DMA）中形成一系列卤代络合物。钴离子在[CoCl_3（溶剂）]^-和[CoCl_4]^-中均为四配位<2->，由反应速率与温度的关系确定了[CoCl_3（溶剂）]^- + Cl^- = [CoCl_4]^ +溶剂的活化焓和熵<2->。活化焓对反应速率起决定性作用，随着活化焓的增加，反应速率变慢。结合基态热力学参数，得到了逆反应的活化焓。结果表明，溶剂化空间效应在活化态下表现得更强，且表现形式与基态不同。在上述反应中，形成五配位的[CoCl_4（溶剂）]^<2->。通过对相关物种从DMF到DMA的转移和在这些溶剂中的活化过程的计算，[CoCl^4（溶剂）]^<2->从DMF到DMA的转移焓估计为26 kJ mol^<-1>，这表明Co^2+-O（DMA）键长变得比Co^2+-O（DMF）键长。另一方面，DMA的活化熵的降低小于DMF，表明DMA的运动自由度的降低小于DMF，这可能是由于Co^&lt;2+&gt;-O（DMA）键长比Co^&lt;2+&gt;-O（DMF）键长长。

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.