KINETICS OF DISSOCIATION OF NICKEL COMPLEXES WITH PENTADENTATE LIGANDS

Abstract

Abstract The synthesis of the pentadentate ligands 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane (BPTAN) and 1,4,7-triazacyclononane-N,N′-diacetate (TCDA) are reported. The nickel(II) complexes of BPTAN and TCDA were synthesized and characterized by their elemental analyses, electrochemistry, and uv-vis spectra. The rate of nickel ion aquation from the BPTAN complex in aqueous solution in the presence of chloride ion was found to follow the rate law, Rate = k1[H+]2[Cl][NiL]/{1 + k2[Cl]}. At 60°C and I = 2.0 M, values for k1 and k2 were determined to be (1.7 ± 0.2) × 10−6 M−3 s−1 and 0.15 ± 0.04 M−1 respectively. For NiTCDA(H2O), the rate law was found to be independent of the chloride ion concentration. The rate law was found to follow the form, Rate = k1[H+][NiL]/(1 + k2H+]). At 60°C and I = 2.0 M, k1 and k2 were found to be equal to 3 M−1 s−1 and 0.90±0.07 M−1, respectively. This work has shown NiBPTAN(H2O)2+ to be one of the most inert nickel complexes to ligand dissociation that has been reported.