Abstract
Rate constants for the formation and dissociation of l-serine complexes of Ni(II), Co(II), Cu(II) and Zn(II) have been determined by the temperature-jump technique. For the overall reaction at 25° and I = 0.1 M, (where M = Ni(II), Co(II), Cu(II) or Zn(II), L = anionic form of the ligand, n = 1, 2, or 3, and charges have been neglected), the results are: Ni(II), k1 = 2.9·104 M−1·sec−1, k−1 = 0.11 sec−1, k2 = 3.4·104 M−1·sec−1, k−2 = 1.6 sec−1, k3 = 3.0·104 M−1·sec−1, k−3 = 28 sec−1; Co(II), k1 = 2.0·106 M−1·sec−1, k−1 = 93 sec−1k2 = 2.0·106 M−1·sec−1, k−2 = 930 sec−1; Cu(II), k1 = 2.5·109 M−1·sec−1, k2 = 5.108k−2 = 150 sec−1; Zn(II)_,.
These results are interpreted in terms of a mechanism in which the rate-determining step for complex formation is the loss of a water molecule from the metal ion's inner coordination sphere. (A possible exception is the reaction sifCuL + L ⇌ CuL2, where changes in the metal ion's configuration may affect the forward rate.) All the dissociative rate constants appear to be influenced by the bond weakening effect of the β-hydroxyl group on the coordinate linkage.