Kinetic studies on the formation of ternary complexes in the reaction of diaquo-nitrilotriacetato-nickelate(II) and diaquo-anthranilato-N, N′-diacetato-nickelate(II) with amino acids in aqueous solution

Summary

Kinetics of formation of ternary complexes from diaquo-nitrilotriacetatonickelate(II), [Ni(nta)(H₂O)₂]⁻, and diaquoanthranilato-N, N-diacetatonickelate(II), [Ni(ada)-(H₂O₂] and amino acids have been studied by a pH indicator method using stopped-flow spectrophotontetry. The results conform to 1/k_obs=1/k+[H⁺]/kK·T_L, where K is the equilibrium constant for the formation of [Ni(A)(-L)(H₂O)]²⁻(A=nta³⁻ or ada³⁻) and k is the specific rate constant for the subsequent rate-determining ring closure leading to [Ni(A) (=L)]²⁻. For the different amino acids, the k values decrease in the sequence: glycine>α-alanine>L-phenylalanine>L-Valine>L-methionine>β-alanine>sarcosine>N,N-dimethylglycine, and areca. 1000 times smaller than the k values for complexation of [Ni)(nta)(H₂O)₂]⁻ with monodentate ligands, such as NH₃ and imidazole. The spread of k values is much less than the pK_a values of the amino acids, and can be accounted for on the basis of the proposed mechanism. The relative rates are enthalpy controlled and high negative ΔS^≠ values are commensurate with ring closure as the rate-determining step.