Kinetics and mechanism of dissociation of ethylenedibiguanidenickel(II) and copper(II) and of bisbiguanide-nickel(II) in acid

Summary

The kinetics of dissociation of the ethylenedibiguanidenickel(II) and copper(II), [M(EndibigH₂)]²⁺, where M = Ni^II or Cu^II, and bisbiguanide-nickel(II), [Ni(BigH)₂]²⁺, complexes in acid media forming the aquo-metal ions and the protonated ligand as the ultimate products have been studied by the stopped-flow technique. The reactions occur in two consecutive steps, the first being faster than the second, forming aquometal ions and protonated ligands as the ultimate products. For each step the rate is acid dependent and may be expressed by: k_x = k ^′_x [H⁺] + k ^″_x [H⁺]², where k_x is the observed rate constant and x = “f” or “s” for fast and slow steps respectively. Both paths (k ^′_x and k ^″_x ) contribute in the ethylenedibiguanidenickel(II) system, whereas k ^′_x paths are virtually absent in the corresponding copper(II) complex; k ^″_x paths are absent in the bisbiguanidenickel(II) system. A likely mechanism involves protonation of the bound ligand which facilitates its dissociation. A comparison of the ΔS^≠ values indicates considerable solvent participation in the transition state, suggesting an assisted dissociative mechanism, which also accounts for the low ΔH^≠ values. The observed lability order is [Ni(EndibigH₂)]²⁺ < [Ni(BigH)₂]²⁺ =ca. [Cu(EndibigH₂)]²⁺, and [Cu(BigH)₂]²⁺ dissociates under comparable conditions at a rate too fast to be measured by the stopped-flow method.