Thermodynamics of formation of urea complexes with manganese(II), nickel(II) and zinc(II) ions in N,N-dimethylformamide

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 0.4M (C(2)H(5))(4) NBF(4) as a constant ionic medium at 25 degrees 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.