Dynamics of ternary complex formation in the reaction of diaquoanthranilato-N,N-diacetatonickelate(II) with 2,2′-bipyridine and 1,10-phenanthroline

Dynamics of ternary complex formation in the reaction of diaquoanthranilato-N, N-diacetatonickelate(II) with 2,2′-bipyridine and 1,10-phenanthroline. $rm Ni(ada)(H_2O)_2^{-}$ $+$ $Lrightleftharpoons Ni(ada)(L)^{-}$ $+$ $2 H_20;$ $- {{d[Ni(ada)^{-}]}over{dt}}$ $=$ $k_f[Ni(ada)^{-}][L]+k_d [Ni(ada)(L)];$ $ ada^{3-}=$anthranilate-N, N-diacetate; and L=bipy or phen. The kinetics of formation of ternary complexes by diaquoanthranilato-N, N-diacetatonickelate(II). [Ni(ada)(H₂O)]⁻ with 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen) have been studied under pseudo-first-order conditions containing excess bipy or phen by stopped-flow spectrophotometry in the pH range 7.1–7.8 at 25°C and λ = 0.1 mol dm⁻³. In each case, the reaction is first-order with respect to both Ni(ada)⁻ and the entering ligand (ie., bipy, phen). The reactions are reversible. The forward rate constants are: $k^{rm Ni(ada)}_{rm Ni(ada)(bipy)}=0.87times10^3{rm dm}^3 {rm mol}^{-1}{rm s}^{-1}$, . $k^{rm Ni(ada)}_{rm Ni(ada)(phen)}=1.87times10^3{rm dm}^3 {rm mol}^{-1}{rm s}^{-1}$; and the reverse rate constants are: $k^{rm Ni(ada)(bipy)}_{rm Ni(ada)}=1.0{rm s}^{-1}$ and $k^{rm Ni(ada)(phen)}_{rm Ni(ada)}=2.0{rm s}^{-1}$. The corresponding stability constants of ternary complex formation are: and , . The observed rate constants and huge drops in stability constants in ternary complex formation agree well with the mechanism in which dissociation of an acetate arm of the coordinated ada³⁻ prior to chelation by the aromatic ligand occurs. The observations have been compared with the kinetics of ternary complex formation in the reaction Ni(ada)⁻ - glycine in which the kinetics involves a singly bonded intermediate, N(ada)((SINGLE BOND)O(SINGLE BOND)N)²⁻ in rapid equilibrium with the reactants followed by a sluggish ring closure step. The reaction with the aromatic ligands conforms to a steady-state mechanism, while for glycine it gets shifted to an equilibrium mechanism. The cause of this difference in mechanistic pathways has been explained. © 1996 John Wiley & Sons, Inc.