Date of Award
Doctor of Philosophy
Dr. Ralph K. Steinhaus
Dr. Robert H. Anderson
Dr. Dean Cooke
Dr. George Lowry
The stopped-flow technique was used to measure rate constants for ternary complex formation involving both NiEDDA and Ni(trien)('2+), and bidentate donors. The reactions of both NiEDDA and Ni(trien)('2+) with bipy, phen, en, and sar were studied. In addition the reaction of NiEDDA and gly was studied. The reactions were of the first-order with respect to either nickel complex, and with respect to enH('+), bipy, and phen. The reactions involving phen and bipy were independent of pH and were reversible. The formation rate constants for the systems containing diamine reactants are as follows:
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For systems containing aromatic diamines, the rate constants for the reverse reactions are: k(,b)(NiE-bipy) = 8.0 x 10('-2) s('-1), k(,b)(NiE-phen) = 3.0 s('-1), k(,b)(NiT-bipy) = 0.18 s('-1), and k(,b)(NiT-phen) = 5.3 s('-1). The reactions of NiEDDA with phen and bipy proceeded at a normal rate. In contrast, the formation rate constants for the reactions of bipy and phen with Ni(trien)('2+) were lower than expected by factors of 36 and 33 respectively. Also, stability constants for (trien)Ni(phen)('2+), and (trien)Ni(bipy)('2+) were lower than expected. For these latter reactions, the reduced formation rates and stabilities are attributed to steric hindrance on the part of trien. The reactions of gly and sar with NiEDDA, and of sar with Ni(trien)('2+) were of the first-order with respect to amino acid at low concentration, and shifted to a zero-order dependence with respect to gly or sar at high amino acid concentration. A two step mechanism was supported by the kinetics. The first step involves an oxygen bonded intermediate in rapid equilibrium with reactants. The equilibrium constants for this step are: K(,1)(NiE-gly) = 1.9 x 10('2) M('-1), K(,1)(NiE-sar) = 93 M('-1), and K(,1)(NiT-sar) = 430 M('-1). The second step, which is rate limiting, involves a slow ring closure; the corresponding rate constants are: k(,2)(NiE-gly) = 44 s('-1), k(,2)(NiE-sar) = 44 s('-1), and k(,2)(NiT-sar) = 68 s('-1). The results show that (1) steric hindrance imposed by a tetradentate ligand coordinated to nickel(II), as well as (2) the type of donor sites on the attacking bidentate ligand, can greatly alter the mechanism of ternary complex formation.
Kolopajlo, Lawrence H., "Kinetics of Ternary Complex Formation Reactions of the Nickel(II) Complexes of Ethylenediaminediacetate and Triethylenetetramine with Bidentate Donors" (1982). Dissertations. 2527.