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The results of a kinetic study of the formation and dissociation of pentacyanoferrate(II) complexes of cysteine (cys), gultathione (glut), penicillamine (pen), and 2-mercaptoethylamine (mea) are reported. Formation rate measurements have been made in the range pH 1–10 and the observed acid dependences are attributed to reaction of various ionic forms of the ligands related by protonation equilibria. For the neutral ligands the rate constants for formation (kL) of complexes with cys, glut, pen, and mea are 330, 370, 350, and 200 dm3 mol–1s–1, respectively, at 25 °C and I= 0.50 mol dm–3(Na[ClO4]). A correlation between the formation rate constants and the ligand charge type is observed; deviations from this relationship are discussed in terms of the location of the charges in the ligand. Rates of ligand release have been measured over the range pH 2.5–11, in the presence of an excess of pyrazine, and a hydrogen-ion dependence observed for each ligand. A decrease in the dissociation rate with increasing pH is attributed to the reduced reactivity of [Fe(CN)5(–SR)]n– with respect to its conjugate acid [Fe(CN)5(HSR)](n–1)–. At pH 5 the rate constants for dissociation (k–L) of cys, glut–, pen, and Hmea+ from [Fe(CN)5L]n– are 0.104, 0.055, 1.34, and 0.102 s–1, respectively. The large value of k–L for penicillamine is associated with the steric effects of the methyl groups.

The kinetic and thermodynamic parameters for the axial-ligand substitution reactions of the bis(acetonitrile) adduct of dirhodium(II) tetraacetate with a variety of phosphines and phosphites in acetonitrile have been measured. The rate-determining formation of the Rh/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/(CH/sub 2/CN)PR/sub 3/ adduct occurs with a rate constant that is independent of the nature of the ligand: k/sub 1/(25.0/sup 0/C) = (1.05 +/- 0.05) x 10/sup 5/ M/sup -1/ s/sup -1/, ..delta..H/sub 1//sup ++/ = 10.9 +/- 0.6 kcal mol/sup -1/, and ..delta..S/sub 1//sup ++/ = 1 +/- 2 cal K/sup -1/ mol/sup -1/. Slightly higher rate constants (1.6 x 10/sup 5/ M/sup -1/ s/sup -1/ at 25.0/sup 0/C) were found for several diphosphines (PPh/sub 2/(CH/sub 2/)/sub n/PPh/sub 2/, n = 2-4) and are attributed to the presence of two ligand coordination sites. A dissociative mechanism is proposed for substitution of the axially coordinated acetonitrile, and the rate parameters are compared with values reported for substitutions of oxygen-donor solvent molecules on the dirhodium(II) complex. The stability constants for the mono- and bisadducts have been measured at 25/sup 0/C with K/sub 1//K/sub 2/ approx. 20. The dependence of K/sub 1/ and K/sub 2/ on the nature of the phosphine or phosphite correlatesmore » well with the sigma-donor strength of the ligand, with ..pi..-acceptor strengths and steric effects being relatively insignificant.« less

The kinetics of the reduction of a nickel(IV) oxime complex, [NiIVL]2+(H2L = 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,15-dione dioxime), by catechol, hydroquinone, and 2-methylhydroquinone (H2Q) have been investigated in aqueous perchlorate media in the pH range 3.5–6. Below pH 4.5 the reaction is monophasic with the NiIV–NiIII step rate-determining. Above pH 4.7 a biphasic process is observed with a rapid NiIV–NiIII reduction followed by a slower reaction of the nickel(III) intermediate. A complex pH dependence is attributed to reaction pathways involving several reductant and oxidant species related by proton equilibria. For the principal reaction pathways involving [NiIVL]2+ and HQ–, rate constants fall in the range ca. 107–108 dm3 mol–1 s–1. With H2Q as reductant, rate constants are ca. 105–106 times smaller. The specific rate parameters compare favourably with the rate constants determined by Marcus theory calculations.

The kinetics of the reduction of a nickel(IV) oxime complex, [NiIVL]2+(H2L = 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,15-dione dioxime), by catechol, hydroquinone, and 2-methylhydroquinone (H2Q) have been investigated in aqueous perchlorate media in the pH range 3.5–6. Below pH 4.5 the reaction is monophasic with the NiIV–NiIII step rate-determining. Above pH 4.7 a biphasic process is observed with a rapid NiIV–NiIII reduction followed by a slower reaction of the nickel(III) intermediate. A complex pH dependence is attributed to reaction pathways involving several reductant and oxidant species related by proton equilibria. For the principal reaction pathways involving [NiIVL]2+ and HQ–, rate constants fall in the range ca. 107–108 dm3 mol–1 s–1. With H2Q as reductant, rate constants are ca. 105–106 times smaller. The specific rate parameters compare favourably with the rate constants determined by Marcus theory calculations.

The results of a kinetic study of the formation and dissociation of pentacyanoferrate(II) complexes of cysteine (cys), gultathione (glut), penicillamine (pen), and 2-mercaptoethylamine (mea) are reported. Formation rate measurements have been made in the range pH 1–10 and the observed acid dependences are attributed to reaction of various ionic forms of the ligands related by protonation equilibria. For the neutral ligands the rate constants for formation (kL) of complexes with cys, glut, pen, and mea are 330, 370, 350, and 200 dm3 mol–1s–1, respectively, at 25 °C and I= 0.50 mol dm–3(Na[ClO4]). A correlation between the formation rate constants and the ligand charge type is observed; deviations from this relationship are discussed in terms of the location of the charges in the ligand. Rates of ligand release have been measured over the range pH 2.5–11, in the presence of an excess of pyrazine, and a hydrogen-ion dependence observed for each ligand. A decrease in the dissociation rate with increasing pH is attributed to the reduced reactivity of [Fe(CN)5(–SR)]n– with respect to its conjugate acid [Fe(CN)5(HSR)](n–1)–. At pH 5 the rate constants for dissociation (k–L) of cys, glut–, pen, and Hmea+ from [Fe(CN)5L]n– are 0.104, 0.055, 1.34, and 0.102 s–1, respectively. The large value of k–L for penicillamine is associated with the steric effects of the methyl groups.

Extended X-ray absorption fine structure (EXAFS) techniques have been used to determine the differences in the metal–ligand bond distances Δd0 for the following couples (couple, Δd0/A) Fe(H2O)2 +/3 +6, 0.13; Fe(CN)4 –/3 –6, 0.01; Fe(phen)2 +/3 +3, 0.00; Co(NH3)2 +/3 +6, 0.22; Co(bpy)3+/2 +, – 0.02; Co(phen)2 +/3 +3, 0.19; Ru(H2O)2 +/3 +6, 0.08; Ru(NH3)2 +/3 +6, 0.04. The Cr(H2O)2 +3 ion has four short and two long Cr—O bonds (2.07 and 2.30 A, respectively) and the average Δd0 for the Cr(H2O)2 +/3 +6 couple is 0.20 A. The rate constant for the Fe(H2O)2 +6/Fe(H2O)3 +6 exchange reaction is 1.1 dm3 mol– 1s– 1 at 25 °C and 0.1 mol dm– 3 ionic strength. The exchange rate is independent of the cation (Li+/Na+) used to maintain constant ionic strength. Exchange rate constants for twelve reactions that span fifteen orders of magnitude are shown to be in excellent agreement with the semi-classical model.