Abstract
The complexation of urea (ur) with manganese(II), nickel(II) and zinc(II) ions has been studied by titration calorimetry in N, N-dimethylformamide (DMF) containing 0.4 M (C 2H 5) 4 NBF 4 as a constant ionic medium at 25°C. The calorimetric data were well explained in terms of the formation of [Mn(ur)] 2+, [Mn(ur) 2] 2+ and [Mn(ur) 4] 2+ for manganese(II), [Ni(ur)] 2+ for nickel(II) and [Zn(ur)] 2+ and [Zn(ur) 2] 2+ for zinc(II), and their formation constants, reaction enthalpies and entropies were determined. The complexation of the nickel(II)-urea system in DMF has also been studied by means of spectrophotometric titration and electronic spectra of individual nickel(II) complexes were determined. On the basis of the stepwise thermodynamic quantities and the individual electronic spectra of the complexes, it is revealed that the [Mn(ur)] 2+, [Mn(ur) 2] 2+, [Ni(ur)] 2+, [Zn(ur)] 2+ and [Zn(ur) 2] 2+ complexes have a six-coordinate octahedral structure, while the [Mn(ur) 4] 2+ complex has a four-coordinate tetrahedral structure.
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