Coordinating properties of pyrrolic and pyridinic nitrogen with metals in aqueous solution. A calorimetric study

Abstract

Enthalpy of complex formation of pyrrole-2-carboxylate ligand with calcium(II), cobalt(II), nickel(II) and copper(II) cations has been determined by direct calorimetry. By means of the equilibrium constants, Gibbs function and entropy were also obtained. The measurements were carried out in aqueous medium at 25ºC and an ionic strength of 1 (NaNO₃). The thermodynamic quantities for pyrrole-2-carboxylate, compared with those for pyridine-2-carboxylate obtained previously, lead to the following conclusions. (A) The enthalpy factors which make the metal-pyridinic ligand complexes rather stable are absent in the case of pyrrolic ligand. The absence is probably due to the electronic structure of the pyrrole aromatic ring. (B) The enthalpy data seem to indicate, for the metal complexes of pyrazole and imidazole, a metal-pyridinic nitrogen bond rather than a metal-pyrrolic nitrogen bond.