The kinetics of formation of several N-salicylideneglycinatometal(II) complexes at 25°, 0.5 M ionic strength, are reported. These systems exhibit reaction paths which are both dependent and independent of the metal ion concentration. The latter paths are identical for all metal ion systems and also to rates measured in the absence of metal. These rates are described by the equation: d[Schiff base]/dt = (kaαH2 + kbαH +kc)[Sal-]-[Gly-], This rate law probably arises from rate-determining attack of glycinate anion on salicylaldehyde anion [kc), and on neutral salicylaldehyde (kb), with the latter reaction exhibiting an acid-catalyzed path (kn). A maximum of three metal-dependent paths was observed. These are considered to arise from rapid preequilibrium between the reactants to give ternary complexes followed by rate-determining reaction of ligands. d[Schiff base]m2+/ dt = {k0 + k1αH + k2[Gly-]}[M2+][Sal-][Gly-] = (kp1 + APHαH)[M(Sal)(Gly)] + KP2[M(Sal)(Gly)2-]. The order of catalytic efficiency of divalent metal ions, as measured by kp1, is Pb ≫ Cd > Mn ~ Mg > Zn ≫ Co, Ni, Cu (negligibly small). No metal ions with partially filled d orbitals exhibit reactivity. This is presumably due to ligand field effects which cause rigid metal-ligand geometries. The order of increasing catalytic efficacy of the other metal ions is essentially that of increasing weakness of the glycine amine group bond to the metal. This effect results in a greater probability of nucleophilic attack on the carbonyl group of salicylaldehyde ligand. Activation of the coordinated carbonyl group by the metal ion appears to be unimportant, but it is indicated that the larger cations are better catalysts than the smaller. The function of the metal ion seems to be better described by the term promnastic1 rather than template. The systems of Mn, Zn Cd, and Pb exhibit an acid-catalyzed promnastic path. It appears that similar acid catalyst mechanisms may obtain for both the promnastic and metal-independent reactions because the values of kph/kp1 are similar and are comparable to the value of ka/kb,- Values of kp2. observed for Zn and Cd systems, were greater by a factor of about 50 than their respective values of kp1. The equilibrium studies showed that the gain in stability resulting from formation of a Schiff base complex from the ternary complex containing independently bound ligands is not dependent on the nature of the metal ion. The formation constant for this process is about 100. Similar behavior is exhibited by analogous ternary systems containing carbonyl compounds other than salicylaldehyde. © 1969, American Chemical Society. All rights reserved.
