Abstract
The rate constants for the formation and dissociation of nickel(II) and cobalt(II) complexes with a- and /3-aminobutyric acids have been determined by the temperature-jump method. Although rate constants for formation of higher order as well as monosubstituted complexes were measured, the most significant results with respect to a comparison between the two different metal ions concern the rate constants (k¡) for the first substi- tution. It was determined that substitution for both nickel(II) and cobalt(II) is faster with -aminobutyric acid than with the ß acid. That is, at 20° an ionic strength = 0.1 M for nickel(II) with -aminobutyrate, k¡ = 1.0 X 104 M-1 sec-1; with /3-aminobutyrate, k¡ = 4.0 X 103 M-1 sec-1. Under the same conditions, for cobalt(II) with
-aminobutyrate, ki = 2.5 X 105 M~l sec-1; with /3-aminobutyrate, k\ = 2.0 X 104 M-1 sec-1. The relative error for these rate constants is ±20%. The rate constants determined for -aminobutyric acid are consistent with a mech- anism in which release of a water molecule from the metal ion’s inner coordination sphere is rate determining. In reaching this conclusion, it is shown that an empirical factor of 1/2, to account for the partial absence of spherical sym- metry in these chelating agents, must be used when comparing these (and the ß) values with rate constants previously determined for other ligands. The slower reactions with /3-aminobutyric acid are explained by the kinetic chelate effect, in which chelate ring closure is the rate-determining step. The steric effect is appreciably greater for cobalt(II) than for nickel(II) because of the inherently greater lability of the former ion.