Steric effects in fast metal complex substitution reactions. II

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The rate constants for the formation and dissociation of nickel(II) and cobalt(II) complexes with α- and β-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 substitution. It was determined that substitution for both nickel(II) and cobalt(II) is faster with α-aminobutyric acid than with the β acid. That is, at 20° an ionic strength = 0.1 M for nickel(II) with α-aminobutyrate, k = 1.0 × 10 M sec ; with β-aminobutyrate, k = 4.0 × 10 M sec . Under the same conditions, for cobalt(II) with α-aminobutyrate, k = 2.5 × 10 M sec ; with β-aminobutyrate, k = 2.0 × 10 M sec . The relative error for these rate constants is ±20%. The rate constants determined for α-aminobutyric acid are consistent with a mechanism 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 symmetry in these chelating agents, must be used when comparing these (and the β) values with rate constants previously determined for other ligands. The slower reactions with β-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. 1 1 1 1 1 4 -1 -1 3 -1 -1 5 -1 -1 4 -1 -1

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Journal of the American Chemical Society

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