KINETIC AND THERMODYNAMIC STABILITY OF NI(III) POLYAZACYCLOTETRADECANE COMPLEXES

The thermodynamic and kinetic stability of Ni(III) in polyazacyclotetr adecane complexes 1-4 was studied as a function of pH, concentration o f indifferent electrolyte (sodium perchlorate), and solvent basicity. The thermodynamic stability was estimated from the changes in formal p otentials of the Ni(II)/Ni(III) redox couple as determined by cyclic v oltammetry. Complexes of Ni(III) were produced by controlled potential electrolysis. Their kinetic stability was studied by UV-Vis spectroph otometry. In every solution investigated the decay of Ni(III) followed first order kinetics. The stability of Ni(III) cyclam decreased with increasing pH and electrolyte concentration. As the basicity of the so lvent increased, the thermodynamic stability of Ni(III) increased in t he series: complex 3 > complex 1 > complex 4 > complex 2. The kinetic stability did not parallel the thermodynamic trend. It is suggested th at the rate determining step in the Ni(III) decomposition process is t he deprotonation of the NH group in the macrocyclic ring. The mechanis m of decomposition is discussed.