The Relationships between the Formation Constants of 2,2′-Bipyridine Metal(II) Complexes and the Donor Numbers of Solvents

The complex formation constants of cobalt(II) or zinc(II) with 2,2′-bipyridine (bpy) were determined spectrophotometrically in various solvents. The values of logK1, logK2, and logK3 for the Co(II) complexes obtained were 4.06±0.06, 3.09±0.04, and 1.15±0.02 respectively in dimethyl sulfoxide, while those of logK1 for the Zn(II)–bpy complexes were 6.27±0.10, 4.14±0.01, 4.71±0.03, and 3.44±0.12 in trimethyl phosphate, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide respectively. It was found that the linear relationships hold between logarithms of the formation constants of the bpy complexes and the donor numbers of the solvents when both the solvated metal ions and the bpy complexes are octahedral in the solvents.