COMPLEXATION OF MANGANESE(II) AND ZINC(II) IONS WITH PYRIDINE, 3-METHYLPYRIDINE AND 4-METHYLPYRIDINE IN DIMETHYLFORMAMIDE

Complexation of manganese(II) and zinc(II) ions with pyridine (py), 3-methylpyridine (3Me-py), and 4-methylpyridine (4Me-py) has been investigated by calorimetry in dimethylformamide (dmf) containing 0.1 mol dm-3 NEt4ClO4 as ionic medium at 25-degrees-C. The formation of [MnL]2+, and [ZnL]2+ and [ZnL]2+ (L = py, 3Me-py or 4Me-py) was revealed, and their formation constants, reaction enthalpies and entropies were determined. The stability of the [ML]2+ (M = Mn or Zn; L = py, 3Me-py or 4Me-py) complexes in dmf linearly increases with increasing basicity of the pyridine derivatives, i.e. a logarithmic plot of the formation constants K, of these complexes in dmf against the protonation constants of the ligands in aqueous solution obeyed a linear free-energy relationship. However, a plot of the formation enthalpies and entropies of the complexes against the ligand basicity was not linear, the values for [M(3Me-py)]2+ (M = Mn or Zn) deviating. For both metal ions in dmf, the formation of the 3Me-py complexes is enthalpically less favourable and entropically more favourable than those of py and 4Me-py. Thus, the presence of a methyl group in the 3 position in pyridine results in a larger steric interaction between the ligand and dmf in the first co-ordination sphere of the metal ions than those involving py and 4Me-py.