COMPLEXATION OF THE ZINC(II) ION WITH 2,2'-BIPYRIDINE AND 1,10-PHENANTHROLINE IN 4-METHYLPYRIDINE AND SOLVATION STRUCTURE OF THE MANGANESE(II), COBALT(II), NICKEL(II), AND ZINC(II) IONS IN 4-METHYLPYRIDINE AND 3-METHYLPYRIDINE

Complexation of the zinc(II) ion with 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) has been calorimetrically studied in 4-methylpyridine (4Me-py) containing 0.1 mol dm(-3) (n-C4H9)(4)NClO4 as a constant ionic medium at 25 degrees C. The formation of [ZnL](2+), [ZnL(2)](2+) and [ZnL(3)](2+) (L = bpy, phen), and their formation constants, reaction enthalpies and entropies were determined. Our EXAFS (extended X-ray absorption fine structure) measurements showed that the solvation structure of the manganese(II), cobalt(II), and nickel(II) ions is six-coordinate octahedral in 4Me-py and 3-methylpyridine (3Me-py), while that of the zinc(ll) ion is four-coordinate tetrahedral in 4Me-py. Since [ZnL(3)](2+) is expected to have an octahedral structure, a tetrahedral-to-octahedral structural change should take place at a certain step of complexation. The thermodynamic parameters, especially reaction entropies, indicate that the structural change occurs at the formation of [Zn(bpy)(2)](2+) and [Zn(phen)](2+).