Solution Equilibria of Binary and Ternary Zinc(II) Halogeno Complexes in N,N-Dimethylacetamide.
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Solution equilibria between zinc(II) and halide ions have been studied in N,N-dimethylacetamide (DMA) by calorimetry at 298 K. Formation constants, enthalpies, and entropies of mononuclear binary complexes, [ZnIICln](2−n)+ (n = 1—4), [ZnIIBrn](2−n)+ (n = 1—3), and [ZnIIIn](2−n)+ (n = 1,2), have been determined. In addition, mixed-ligand complexes have been detected in ternary Zn–Cl–Br and Zn–Br–I systems: [ZnClBr], [ZnCl2Br]−, [ZnClBr2]−, and [ZnBrI]. Complexation is more favorable and more exothermic in the order Cl > Br > I. Formation of the monohalogeno complexes is particularly endothermic and entropy-productive, suggesting that geometric transition from octahedral to tetrahedral is complete at this step, i.e., all the halogeno complexes are tetrahedral. Thus, the transition occurs earlier in DMA than in N,N-dimethylformamide (DMF). Compared with DMF, complexation in DMA is more favorable and more exothermic exclusively at the first and second steps, and little difference is found for the later steps. This is attributed to the steric hindrance of DMA molecules, which is crucial in octahedral coordination structures but not important in tetrahedral geometry. Among the first-row bivalent transition metal ions, ZnII is exceptional in its ΔHβ4° and ΔSβ4° values of overall formation of the tetrachloro complex, consistent with the existence of a solvation equilibrium: [Zn(dma)6]2+ <SUB></SUB> [Zn(dma)4]2+ + 2dma.
- 公益社団法人 日本化学会の論文
公益社団法人 日本化学会 | 論文
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