COMPLEXATION REACTIONS OF COBALT (2) AND NICKEL (2) WITH GLYCYLSARCOSINE

The kinetics of complexation of cobalt(II) and nickel(II) with glycylsarcosine have been studied at 25° and an ionic strength of 0.1 M using the temperature-jump method. Only the glycylsarcosinate anion is an attacking form of this ligand in the pH range investigated. The reactions studied are of the form MLn-1(3-n)+ + L- ⇌k-nkn MLn(2-n) + with n = 1 or 2 and M = Co or Ni. The rate constants for cobalt(II) are k1 = 4.6 × 105 M-1 sec-1, k-1 = 75 sec-1, k2 = 8.0 × 105 M-1 sec-1, and k-2 = 300 sec-1. For nickel, the rate constants are k1 = 2.0 × 103 M-1 sec-1, k-1 = 7.2 × 10-2 sec-1, k2 = 8.0 × 103 M-1 sec-1, and k-2 = 1.8 sec-1. These results have been compared with amino acid and peptide chelating agents having no substituents on the amine or peptide nitrogen. The presence of a methyl group on the peptide nitrogen in glycylsarcosine appears to have no effect on the kinetics of formation of the monosubstituted cobalt complex or nickel complex. The presence of the methyl group plays a role for the cobalt ion by enhancing the rate of the reaction CoL+ + L- (formation of the bis complex) relative to the formation of the monocomplex more so for glycylsarcosine than for other homologous ligands. For nickel(II), the relative rate of enhancement is the same as for homologous ligands. © Copyright, 1969, by the American Chemical Society.