Kinetics and Mechanism of Metal Substitution and the Irving-Williams Series: Anion-Catalyzed Substitution of Nickel for Copper in Cu(amben) [=(N,N'-Ethylenebis(2-aminobenzaldiminato))copper(II)].

Visible spectrophotometry was used to study the kinetics of metal substitution in the system Cu(amben)/Ni(DMF)(6)(ClO(4))(2)/R(4)NX in DMF at 298 K and I = 0.2 M (Bu(4)NPF(6)) (H(2)amben = N,N'-ethylenebis(2-aminobenzaldimine); X = Br(-)(), SCN(-), Cl(-); DMF = N,N-dimethylformamide). The rate of nickel for copper substitution is markedly affected by the nature of the anion X, the order of reactivity being ClO(4)- < Br(-) < SCN(-) << Cl(-). For X = Cl(-) and under excess conditions [(Cu(amben)] << [Ni(DMF)(6)(ClO(4))(2)], [BzEt(3)NCl]), the overall process is triphasic and multiwavelength analysis in the range 350-900 nm yields the experimental rate constants k(obsd)(1), k(obsd)(2), and k(obsd)(3). Rate constant k(obsd)(1) is nickel and chloride dependent, whereas k(obsd)(2) and k(obsd)(3) are not. For the rate of disappearance of Cu(amben), v = -d[Cu(amben)]/dt, it is found that v ~ [Cl](tot)(2) at [Ni(DMF)(6)(ClO(4))(2)](0) = constant and v ~ [Ni](tot)(3) at [nickel]:[chloride] = 1:2 ([Cl](tot) and [Ni](tot) refer to the total concentration of chloride and nickel, respectively). These dependencies suggest a second-order rate law, v = k(1)[Cu(amben)]["NiCl(2)"], for the first step, with k(1) = 64 +/- 8 M(-)(1) s(-)(1) at 298 K. In view of the ligand properties of Cu(amben) it is concluded that the species "NiCl(2)" is tetrahedral Ni(DMF)(2)Cl(2) which reacts with Cu(amben) as ligand to form a binuclear adduct in the first step. It is suggested that the observed rate effects of the anions X are linked with their ability to favor the formation of nickel species with reduced coordination number, such as four-coordinate Ni(DMF)(2)X(2). The mechanism of the overall process of nickel for copper substitution in Cu(amben) is discussed. The results of the analysis of the visible spectra of Cu(amben) and Ni(amben) and of MO calculations on the charge distribution in these complexes are reported.