Outer Sphere Association Complex Research Articles

Thermodynamic and Kinetic Studies on Novel Platinum(II) Complex Containing Bidentate N,N-Donor Ligands in Ethanol-Water Medium

Kinetic and mechanistic investigations have been made on the displacement of the two aqua molecules from the complex 1 i.e., [Pt(2,2′-bipyridine)(H2O)2]2+ represented as {Pt(bpy)} (bpy = 2,2′-bipyridine) at pH 4.0. All the substitution reactions have been monitored at 264, 284 & 317 nm respectively where the spectral difference between the reactant and product is maximum. Two consecutive reaction steps were observed for the substitution of aqua molecules with some didentate N- and N-donor ligands namely, dimethylglyoxime (L1H), 1, 2-cyclohexanedionedioxime (L2H) and α-furildioxime (L3H) in ethanol-water medium using variable-temperature and stopped-flow spectrophotometry. Among the two steps, the former is ligand dependent and the later is ligand independent, where chelation observed. All rate and activation parameters are consistent with associative substitution mechanisms. The thermodynamic parameters were also calculated, which gives a negative DG0 value at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The products of the reaction have been characterized with the help of IR and ESI-MS spectroscopic analysis.

Kinetics and mechanism for ligand substitution reactions of some square-planar platinum(II) complexes: Stability and reactivity correlations

ABSTRACTA brief overview of mechanistic studies on the reactions of cis-[Pt(dap)(H2O)2]2+ (1) and cis-[Pt(pipen)(H2O)2]2+ (2) (where dap and pipen are 1,3-diaminopropane and 2-aminomethylpiperidine respectively) with 2-thiouracil (LH), a nitrogen and sulfur-donor bio molecule, is presented. All the substitution reactions have been monitored at 327(for 1) and 326 (for 2) nm respectively where the differences in abosorbances between the reactants and products are maximum. At pH 4.0, the kinetics shows two consecutive processes, i.e., they show a non linear dependence on the concentration of ligand: first process is [ligand] dependent but the second step is [ligand] independent. The activation parameters and the thermodynamic parameters were also calculated, which supports the spontaneous formation of an outer sphere association complex. The product of this reaction has been characterized with the help of Job's method, IR and ESI-MS spectroscopic analysis.The purpose of this work is to improve the understanding of the mechanism of action of Pt(II) complexes as potential anti-tumour drugs in the human body.

Kinetic and mechanistic studies on the substitution of aqua ligands from diaquaethylenediamineplatinum(II) ion by vicinal dioximes

Kinetics of aqua ligand substitution from diaquaethylenediamineplatinum(II) ion by three vicinal dioximes, namely dimethylglyoxime, 1,2-cyclohexanedione dioxime, and α-furil dioxime have been studied spectrophotometrically in the 40–55 °C temperature range. All the above substitution reactions have been monitored at 297, 298, and 314 nm where the differences in absorbances between the reactant and products are maximum. At pH 4.0, all the three reactions have been found to proceed via two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentrations of ligands: first step is [ligand] dependent but the second step is [ligand] independent. The rate constants for the processes are k 1 ~ 10−3 and k 2 ~ 10−5 s−1. The activation parameters, calculated from Eyring plots, suggest an associative mechanism for the interaction processes. From the temperature dependence of the outer sphere association equilibrium constants (K E) for the first step, the thermodynamic parameters were also calculated, which gives a negative Δ $$ G_{ 1}^{0} $$ value at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex in the first step. The products of the reactions have been characterized with the help of Job’s method, IR and ESI–MS spectroscopic analysis.

Mechanistic Aspects of Ligand Substitution on the Hydroxopentaaquarhodium(III) Ion in Aqueous Solution by Sulfur‐Containing Bioactive Ligands

ABSTRACTThe kinetics of the interactions between three sulfur‐containing ligands, thioglycolic acid, 2‐thiouracil, glutathione, and the title complex, have been studied spectrophotometrically in aqueous medium as a function of the concentrations of the ligands, temperature, and pH at constant ionic strength. The reactions follow a two‐step process in which the first step is ligand‐dependent and the second step is ligand‐independent chelation. Rate constants (k1 ∼10−3 s−1 and k2 ∼10−5 s−1) and activation parameters (for thioglycolic acid: ΔH1≠ = 22.4 ± 3.0 kJ mol−1, ΔS1≠ = −220 ± 11 J K−1 mol−1, ΔH2≠ = 38.5 ± 1.3 kJ mol−1, ΔS2≠ = −204 ± 4 J K−1 mol−1; for 2‐thiouracil: ΔH1≠ = 42.2 ± 2.0 kJ mol−1, ΔS1≠ = −169 ± 6 J K−1 mol−1, ΔH2≠ = 66.1 ± 0.5 kJ mol−1, ΔS2≠ = −124 ± 2 J K−1 mol−1; for glutathione: ΔH1≠ = 47.2 ± 1.7 kJ mol−1, ΔS1≠ = −155 ± 5 J K−1mol−1, ΔH2≠ = 73.5 ± 1.1 kJ mol−1, ΔS2≠ = −105 ± 3 J K−1 mol−1) were calculated. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. The products of the reactions have been characterized from IR and ESI mass spectroscopic analysis. A rate law involving the outer sphere association complex formation has been established as urn:x-wiley:05388066:media:kin20873:kin20873-math-0002

The Reactivity of vic-dioximes Towards the [(H2O)(tap)2RuORu(tap)2(H2O)]2+ Ion {tap = 2-(m-tolylazo)pyridine} at Physiological pH

Kinetics of aqua ligand substitution from [(H2O)(tap)2RuORu(tap)2(H2O)]2+ {tap = 2-(m-tolylazo)pyridine}, by three vicinal dioximes, namely dimethylglyoxime (L1H), 1,2-cyclohexanedione dioxime (L2H) and α-furil dioxime (L3H), have been studied spectrophotometrically in the 35–50 °C temperature range. The reaction was monitored at 560 nm where the absorbance between the reactant and product is at a maximum. At pH 7.4, the reaction has been found to proceed via two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of ligands: the first process is [ligand] dependent but the second step is [ligand] independent. The rate constants for the processes are: k 1 ~ 10−3 s−1 and k 2 ~ 10−4 s−1. The activation parameters, calculated from Eyring plots, suggest an associative mechanism for the interaction process. From the temperature dependence of the outer sphere association equilibrium constants, the thermodynamic parameters were also calculated, which give negative ΔG° values at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterized with the help of IR and ESI-mass spectroscopic analysis.

Mechanistic Aspects of Ligand Substitution on cis-diaqua-chloro-tris(Dimethyl Sulfoxide) Ruthenium(II) Complex by Adenosine and Cytidine in Aqueous Medium

The interaction of the cis-diaqua-chloro-tris(dimethyl sulfoxide)ruthenium(II) complex with selected nucleosides such as adenosine (L1H) and cytidine (L2H) has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand] and temperature. The reaction has been monitored at 295 nm where the difference in spectra between the reactant and product is a maximum. At pH 5.0, the interaction with the two nucleosides shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of the nucleosides: both steps are [ligand] dependent. The rate constants for the processes are: k1 ∼ 10–3 s–1 and k2 ∼ 10–5 s–1. The activation parameters were calculated from Eyring plots. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. From the temperature dependence of the outer sphere association equilibrium constant, the thermodynamic parameters were also calculated, which gives a negative Δ G° value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterised by IR and ESI-mass spectroscopic analysis.

Kinetics and mechanism of interaction of some bioactive ligands with cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II) in aqueous medium

The substitution reaction of cis-[Pt(cis-dach)(H2O)2]2 + (where ‘dach’ is cis-1,2-diaminocyclohexane) with 2-thiouracil (S, N), 1,2-cyclohexanedionedioxime (N, N) and acetylacetone (O, O) were studied in aqueous solution in 0.10 M NaClO4 under pseudo-first order conditions as a functions of concentration, pH and temperature using UV-Vis spectrophotometry. The substitution reaction proceeds via rapid outer sphere association complex formation, followed by two slow consecutive steps. The first of these involves ligand-assisted deaquation, while second involves chelation as the second aqua ligand is displaced. The association equilibrium constant (KE) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. The rate constants increase with increasing ligand concentration and the evaluated activation parameters for all reactions suggest an associative substitution mechanism for both the aqua ligand substitution processes. The product of the reaction has been characterized by IR, NMR and ESI-MS spectral analysis; which throws more light on the mechanistic behaviour of platinum(II) antitumour complexes. The interaction of cis-[Pt(cis-dach)(H2O)2]2 + (where ‘dach’ is cis-1,2- diaminocyclohexane) with 2-thiouracil (S, N), 1,2-cyclohexanedionedioxime (N, N) and acetylacetone (O, O) studied in aqueous solution; the products have been characterized by IR, NMR and ESI-MS analysis.

Displacement of aqua ligands from the hydroxopentaaquarhodium(III) ion by 1-hydroxybenzotriazole (HOBt): A kinetic and mechanistic approach

The kinetics of the reaction of HOBt with [Rh(H2O)5(OH)]2 + has been studied spectrophotometrically in aqueous medium as a function of [Rh(H2O)5OH2 + ], [HOBt], pH and temperature. At pH 4.3, the reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyrings equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation. The product of the reaction has been characterized by IR and ESI-mass spectroscopic analysis. The kinetics of the reaction of HOBt with [Rh(H2O)5(OH)]2+ has been studied spectrophotometrically in aqueous medium as a function of [Rh(H2O)5OH2+], [HOBt], pH and temperature. At pH 4.3, the reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyrings equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation.

Kinetic studies on substitution of cis-diaqua-chloro-tris-(dimethyl sulphoxide)-ruthenium(II) complex with some dipeptides in aqueous medium

The kinetics of interaction between cis-[RuCl(Me2SO)3(H2O)2] + and some selected dipeptides such as glycyl glycine(gly gly), glycyl alanine(Gly-L-ala) and glycyl-L-leucine(gly leu) has been studied spectrophotometrically as a function of [RuCl(Me2SO)3(H2O)\(_{2}^{+}\)], [dipeptide] and temperature at a particular pH(5.0), where the substrate complex exists predominantly as a diaqua species (in aqueous solution) and dipeptides as the zwitter ions. The reaction has been found to proceed via two distinct consecutive steps i.e., it shows a non-linear dependence on the concentration of dipeptide: first process is dependent and the second step is independent of ligand concentration respectively. The rate constants for the processes are: k\(_{1} \sim 10^{-3}\)s − 1 and k\(_{2} \sim 10^{-5}\)s − 1. The activation parameters were calculated from Eyring plots suggests an associative mechanism for the interaction process. From the temperature dependence of the outer sphere association equilibrium constants, the thermodynamic parameters were also calculated, which gives a negative ΔG0 value at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The kinetics of interaction between cis-[RuCl(Me2SO)3(H2O)2]+ and some glycyl-containing dipeptides such as glycyl glycine(gly gly), glycyl alanine(gly ala) and glycyl-L-leucine(gly leu) has been studied spectrophotometrically as a function of [RuCl(Me2SO)3(H2O)2]+, [dipeptide] and temperature at a particular ph(5.0), where the substrate complex exists predominantly as a diaqua species (in aqueous solution) and dipeptides as the zwitter ions. The reaction has been found to proceed via two distinct consecutive steps: first process is [ligand] dependent whereas the second step is [ligand] independent. The rate constants for the processes are: k1~10−3s−1 and k2~10−5s−1.

Kinetics and Mechanism of the Interaction of Glycyl-L-Leucine with [(H2O)(Tap)2Ruoru(Tap)2(H2O)]2+ Ion at Physiological Ph

The interaction of glycyl-L-leucine (L-I/H) with [(H2O) (tap)2RuORu(tap)2(H2O)]2+ (tap=2-(m-tolylazo) pyridine) has been studied spectrophotometrically in aqueous medium as a function of [(H2O) (tap)2RuORu(tap)2(H2O)2+], [glycyl-L-leucine], pH and temperature. The reaction was monitored at 600 nm where the spectral difference between the reactant and product is a maximum. At pH 7.4, the interaction with glycyl-L-leucine shows two parallel steps, i.e. it shows a non-linear dependence on the concentration of glycyl-L-leucine; both processes are ligand dependent. The rate constants for the processes are: kl 10−3 s 1 and K2∼10−5 s−1. The activation parameters calculated from Eyring plots are: Δ H#1 = 15.8∓0.6kJ mol−1, A Sf = −232 + 2J K 1 mol−1, AH2# =31.7∓3.5kJmol−1, AS* = −216∓ 10J K−1 mol−1. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. from the temperature dependence of the outer sphere association equilibrium constant, the thermodynamic parameters were also calculated, which gave a negative AG° value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterized from IR and ESI-mass spectroscopic analysis.

Kinetic and Mechanistic Study of the Interaction of Diethyldithiocarbamate Anion with Diaquaethylenediamineplatinum(Ii) Ion in Aqueous Medium

The interaction of diethyldithiocarbamate (Et2DTC) with diaquaethylenediamineplatinum(II) has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], temperature and pH at constant ionic strength. The reaction proceeds via rapid outer-sphere association complex formation followed by two consecutive steps: the first is ligand dependent and second is assigned to intramolecular ring closure. The low enthalpy and negative entropy of activation indicate an associative mode of activation for both steps.

Mechanistic aspects of ligand substitution on [(H2O)(tap)2RuORu(tap)2(H2O)]2+ ion {tap = 2-(m-tolylazo)pyridine} by some amino acids in aqueous medium at physiological pH

The interaction of the title complex with selected amino acids such as glycine (L1H), l-valine (L2H), and l-leucine (L3H) has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], and temperature. The reaction has been monitored at 600 nm, where the spectral difference between the reactant and product is maximum. At pH 7.4, the reaction has been found to proceed via two distinct consecutive steps, i.e., it shows a nonlinear dependence on the concentration of ligands: The first process is [ligand] dependent, but the second step is [ligand] independent. The rate constants for the processes are k1∼10−3 s−1 and k2∼10−4 s −1. The activation parameters were calculated from Eyring plots. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. From the temperature dependence of the outer sphere association equilibrium constant, the thermodynamic parameters were also calculated, which gives a negative Δ Go value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterized with the help of conductance measurement, IR, NMR, and ESI-mass spectroscopic analysis. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 612–623, 2012

Kinetic and mechanistic studies on the reaction of thiosemicarbazide with [(H2O)(tap)2RuORu(tap)2(H2O)]2+ {tap = 2-(m-tolylazo)pyridine}

The interaction of thiosemicarbazide with the title complex has been studied spectrophotometrically in aqueous medium as a function of [complex], [thiosemicarbazide], pH and temperature at constant ionic strength. At pH 7.4, the reaction shows two distinct paths; both of which are [thiosemicarbazide] dependent. A parallel reaction scheme fits well with the experimental findings. An associative interchange mechanism is proposed for both the paths; the activation parameters calculated from Eyring plots are ΔH 1 ≠ = 14.2 ± 0.8 kJ mol−1, ΔS 1 ≠ = −241 ± 2 JK−1 mol−1, ΔH 2 ≠ = 30.8 ± 1.4 kJ mol−1 and ΔS 2 ≠ = −236 ± 4 JK−1 mol−1. From the temperature dependence of the outer sphere association complex equilibrium constants, the thermodynamic parameters calculated are ΔH 1 ° = 34.25 ± 1.9 kJ mol−1, ΔS 1 ° = 146 ± 6 J K−1 mol−1 and ΔH 2 ° = 9.4 ± 1.1 kJ mol−1, ΔS 2 ° = 71 ± 3 JK−1 mol−1, which gives a negative ΔG° at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex.

Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with l-Arginine in aqueous solution

The kinetics of the aqua ligand substitution from hydroxopentaaquarhodium(III) ion, [Rh(H2O)5(OH)]2+, by l-Arginine has been studied spectrophotometrically as a function of Arginine concentration, and temperature, at pH 4.3. The reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyring’s equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation. The product of the reaction has been characterized by conductivity measurement and IR spectroscopic analysis.

Kinetics and mechanism of aqua ligand substitution from cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II)perchlorate by diethyldithiocarbamate anion in aqueous medium

The kinetics of the interaction of diethyldithiocarbamate (Et2DTC) with [Pt(dach)(H2O)2]2+ (dach = cis-1,2-diaminocyclohexane) have been studied spectrophotometrically as a function of [Pt(dach)(H2O)22+], [Et2DTC] and temperature at a particular pH (4.0). The reaction proceeds via rapid outer sphere association complex formation followed by two slow consecutive steps. The first step involves the transformation of the outer sphere complex into an inner sphere complex containing a Pt–S bond and one aqua ligand, while the second step involves chelation when the second aqua ligand is replaced. The association equilibrium constant KE and two rate constants k1 and k2 have been evaluated. Activation parameters for both the steps have been calculated (∆H1# = 66.8 ± 3.7 kJ mol−1, ∆S1# = −81 ± 12 JK−1 mol−1 and ∆H2# = 95.1 ± 2.8 kJ mol−1, ∆S2# = −34.4 ± 9.1 JK−1 mol−1). The low enthalpy of activation and negative entropy of activation indicate an associative mode of activation for both the steps.

A Kinetic and Mechanistic Study on Substitution of Aqua Ligands from cis-diaqua-bis-(bipyridyl)-ruthenium(II) Complex by Thiosemicarbazide in Aqueous Medium

The kinetics of the interaction of thiosemicarbazide with cis-[Ru(bipy)2(H2O)2]2+ (bipy = α α′-bipyridyl) have been studied spectrophotometrically as a function of [Ru(bipy)2(H2O) 2 2+ ], [bipyridyl] and temperature, at a particular pH (4.8), where the substrate complex exists predominantly as the diaqua species and thiosemicarbazide as the neutral ligand. The reaction proceeds via an outer sphere association complex formation, followed by two slow consecutive steps. The first is the conversion of the aforementioned complex into the inner sphere complex, and the second step involves the entrance of another thiosemicarbazide molecule in the coordination zone of Ru(II) whereby, in each step, an aqua ligand is replaced. The association equilibrium constant (KE) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. Activation parameters have been calculated for both steps using the Eyring equation (ΔH 1 # = 25.37±1.6 kJ mol−1, ΔS 1 # = −215.48 ± 4.5 J K−1 mol−1, ΔH 2 # = 24.24 ± 1.1 kJ mol−1, ΔS 2 # = −207.14 ± 3.0 J K−1 mol−1). The low enthalpy of activation and large negative value of entropy of activation indicate an associative mode of activation for both aqua ligand substitution processes. From the temperature dependence of KE, the thermodynamic parameters calculated are: ΔH0 = 10.75±0.54 kJ mol−1 and ΔS0 = 84.67 ± 1.75 J K−1 mol−1, which give a negative ΔG0 value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex prior to the first step.

Kinetic and mechanistic studies on the interaction of thymidine with the hydroxopentaaquarhodium(III) ion

The interaction of thymidine, a nucleoside, with hydroxopentaaquarhodium(III), [Rh(H2O)5(OH)]2+ ion in aqueous medium is reported and the possible mode of binding is discussed. The kinetics of interaction between thymidine and [Rh(H2O)5OH]2+ has been studied spectrophotometrically as a function of [Rh(H2O)5OH2+], [thymidine], pH and temperature. The reaction has been monitored at 298 nm, the λmax of the substituted complex, and where the spectral difference between the reactant and product is a maximum. The reaction rate increases with [thymidine] and reaches a limiting value at a higher ligand concentration. From the experimental findings an associative interchange mechanism for the substitution process is suggested. The activation parameters (ΔH‡=47.8 ± 5.7 kJ mol−1, ΔS‡=−173 ± 17 J K−1 mol−1) supports our proposition. The negative ΔG0 (−13.8 kJ mol−1) for the first equilibrium step also supports the spontaneous formation of the outer sphere association complex.

Kinetics and mechanism of the interaction of azide with [(H2O)(tap)2RuORu(tap)2-(H2O)]2+ion at physiological pH

The title reaction has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], pH and temperature at constant ionic strength. At the physiological pH (7.4) the interaction with azide shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of N3−; both processes are [ligand]-dependent. The rate constant for the processes are: k1∼10−3 s−1 and k2∼10−5 s−1. The activation parameters calculated from Eyring plots are: ΔH1‡ = 14.8 ± 1 kJ mol−1, ΔS1‡ = −240 ± 3 J K−1 mol−1, ΔH2‡ = 44.0 ± 1.5 kJ mol−1 and ΔS2‡ = −190 ± 4 J K−1 mol−1. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for the interaction process. From the temperature dependence of the outersphere association equilibrium constant, the thermodynamic parameters calculated are: ΔH10 = 4.4 ± 0.9 kJ mol−1, ΔS10 = 64 ± 3 J K−1 mol−1 and ΔH20 = 14.2 ± 2.9 kJ mol−1, ΔS20 = 90 ± 9 J K−1 mol−1, which gives a negative ΔG0 value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex.

Kinetics of substitution of aqua ligands from cis-diaqua(ethylenediamine)platinum(II) perchlorate by DL-penicillamine in aqueous medium

The kinetics of the interaction of DL-penicillamine with [Pt(en)(H2O)2]2+ have been studied spectrophotometrically as a function of [Pt(en)(H2O)2]2+, [DL-penicillamine] and temperature at pH 4.0. The reaction proceeds via rapid outer sphere association complex formation, followed by two slow consecutive steps. The first is the conversion of the aforementioned complex into the inner sphere complex and the second is the slower chelation step whereby another aqua ligand is replaced. The association equilibrium constant (KE) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. Activation parameters have been calculated for both steps using the Eyring equation (ΔH1‡ = 46.5 ± 5.0 kJ mol−1, ΔS1‡ = − 143.0 ± 15.0 J K−1 mol−1, ΔH2‡ = 44.3 ± 1.3 kJ mol−1, ΔS2‡ = −189.0 ± 4.2 J K−1 mol−1). The low enthalpy of activation and large negative entropy of activation values indicate an associative mode of activation for both aqua ligand substitution processes.

The Reaction of the Aquopentamminechromium(III) Complex Ion in Aqueous Solution. I. Substitution Reaction of a Coordinated Water with a Chloride Ion

Abstract The substitution reaction of aquopentamminechromium(III) ion in which a coordinated water molecule is replaced by a chloride ion was investigated spectrophotometrically. The reaction proceeds according to the second order rate law, d[[Cr(NH3)5Cl]2+]/dt=\overrightarrowk2[[Cr(NH3)5OH2]3+]. [Cl−], and the rate constant varies regularly with the ionic strength of solution according to the Debye-Hückel type equation. The formation of outer sphere association complex in the solution was detected from the change in ultraviolet absorption. From these results the ratedetermining process of this reaction was inferred to be a unimolecular process through the outer sphere association complex. The experimental activation energy and the frequency factor of this reaction were determined to be 24.6 kcal.mol−1 and 1.2×1014l. mol−1min−1 (ω∼0.52), respectively. The aquation of chloropentamminechromium(III) ion was also studied in the solutions of various ionic strengths and the reaction rate was found to vary to some extent with ionic environment.