Summary
Acid catalysed dissociation of the copper(II) and nickel(II) complexes (ML2+ of the quadridentate macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene (L) has been studied spectrophotometrically. Both complexes dissociate quite slowly with the observed pseudo-first order rate constants (kobs) showing acid dependence; for the nickel(II) complex (kobs)=kO+kH[H+], the ko path is however absent with the copper(II) complex. At 60°C (I=0.1M) the kH values areca 10−4 M−1 s−1 for both complexes; k CuH /k NiH =ca. 3.9, comparable to some other square-planar complexes of these metal ions. The rate difference is primarily due to ΔH≠ values [copper(II) complex, 29.4±0.5 kJ mol−1; nickel(II) complex, 35.6±1.5 kJ mol−1] with highly negative ΔS≠ values [for copper(II), −215.5 ±6.1 JK−1 mol−1 and for nickel(II), −208.1 ±5.6 JK−1 mol−1] which are much higher than the entropy of solvation of Ni2+ (ca. −160 JK−1 mol−1) and Cu2+ (ca. −99 JK−1 mol−1) ions; significant solvation of the released metal ions and the ligand is indicated.
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Pondit, A.K., Das, A.K., Gangopadhyay, S. et al. Studies on the kinetics and mechanism of dissociation of copper(II) and nickel(II) complexes of the macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene in perchloric acid media. Transition Met Chem 13, 437–439 (1988). https://doi.org/10.1007/BF01043706
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DOI: https://doi.org/10.1007/BF01043706