| Research Abstract |
Metal ions exist as solvated complexes in aprotic donor solvents such as dimethylformamide (DMF), dimethylacetamide (DMA), and hexamethylphosphoric triamide (HMPA). The coordination structure and intramolecular interactions of the solvated metal complexes are largely different from those of hydrated metal complexes, and the difference play a key role in thermodynamic and kinetic complexation aspects in these aprotic solvents. In the present study, we particularly focused our attention on solvation steric effects in some aprotic donor solvents. As kinetic aspects on metal complexation in nonaqueous solution have not been revealed yet, we newly introduced a stopped-flow spectrophotometer, and kinetic analyzes have been carried out for the Ni-Cl-bpy systems for which precise thermodynamic analyzes have been accomplished.
It is found that in DMF the rate of formation of the tris-bipyridine nickel (II) complex is reduced when chloride ions coexist in solution. In the Ni-Cl-bpy ternary system
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, the [NiCl (bpy)_2]^+ and [NiCl_2bpy_2] complexes are significantly formed and the pass through these complexes was found to be slow compared to the pass through [NiCl (bpy)_2]^<2+>, suggesting a presence of a specific interaction within the intermediate [NiCl (bpy)_3]^+, in which one bpy coordinates as monodentate. Solvation steric effect exhibited a strong relationship to the ionic radius of metal ions. Halogeno complexation of beryllium, aluminum, cadmium and manganese ions were examined in DMF and DMA,and their thermodynamic parameters were compared. The four-coordinated small beryllium ion show no appreciable steric effect. One the other hand, relatively large and six-coordinated cadmium ion exhibits a significant steric effect. Even larger lanthanide ions show also significant steric efffect, and it is indicated by EXAFS that the coordination number in DMA is reduced by one compared with that in DMF.It is also indicated by ^<89>Y NMR that in DMF-DMA mixtures, the coordination number reduced gradually with increasing DMA content. Less
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