Anation Rate Constants Research Articles

Aquation and Anation Kinetics of Rhenium(I) Dicarbonyl Complexes: Relation to Cell Toxicity and Bioavailability.

The aquation reactions of four rhenium(I) dicarbonyl complexes, [Re(CO)2(NN)(PR3)(Cl)], where NN = 1,10-phenanthroline (Phen) and 2,9-dimethyl-1,10-phenanthroline (DMPhen) and PR3 = 1,3,5-triaza-7-phosphaadamantane (PTA) and 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA). Additionally, the anation reactions of the corresponding aqua complexes with Cl- were investigated. Single crystals of [Re(CO)2(DMPhen)(PTA)(Cl)]·DMF and [Re(CO)2(DMPhen)(DAPTA)(Cl)] were obtained, and their structures were determined using X-ray diffraction. The Re-Cl interatomic distances are 2.4991(13) and 2.4922(6) Å, respectively, indicating a mild trans influence effect of the phosphine ligands. The rate constants, kaq, for the aquation reactions of these complexes spanned a range of (3.7 ± 0.3) × 10-4 to (15.7 ± 0.3) × 10-4 s-1 with the two Phen complexes having rate constants that are 2.5 times greater than those of the DMPhen complexes at 298 K. Similarly, the second-order anation rate constants (kCl) of the resulting aqua complexes, [Re(CO)2(NN)(PR3)(H2O)]+, with Cl- ions at 298 K varied between (2.99 ± 0.05) × 10-3 and (6.79 ± 0.09) × 10-3 M-1 s-1. Likewise, these rate constants for the Phen complexes were almost 2 times faster than those of the DMPhen complexes. The pKa values of the four aqua complexes were determined to be greater than 9.0 for all of the complexes with [Re(CO)2(Phen)(PTA)(H2O)]+ having the highest pKa value of 9.28 ± 0.03. From the pKa values and the ratios of the aquation and anation rate contants, which give thermodynamic Cl- binding constants, the speciation of the rhenium(I) complexes in blood plasma, the cytoplasm, and the cell nucleus were estimated. The data suggest that the aqua complexes would be the dominant species in all three environments. This result may have important implications on the potential biological activity of these complexes.

Kinetics and Mechanism of the Interaction of Di-μ-hydroxobis(1,10-phenanthroline)dipalladium(II) Perchlorate with Thioglycolic Acid and Glutathione in Aqueous Solution

The kinetics of interaction between di-μ-hydroxobis(1,10-phenanthroline)dipalladium(II) perchlorate and thioglycolic acid and with glutathione has been studied spectrophotometrically in aqueous medium as a function of the complex concentration as well as the ligand concentrations, pH, and temperature at constant ionic strength. The observed pseudo-first-order rate constants k obs (s−1) obeyed the equation k obs = k 1[Nu] (Nu = nucleophile). At pH = 6.5, the interaction with thioglycolic acid shows two distinct consecutive steps and both steps are dependent on the concentration of thioglycolic acid. The rate constants for the process are: k 1 ≈ 10−5 s−1 and k 2 ≈ 10−3 dm3 · mol−1 · s−1. The association equilibrium constant (K E) for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The other bio-active ligand, glutathione, showed a single step reaction depending on [ligand] with a second-order anation rate constant: the 102 (k 2) values are (61.72, 79.20, 109.24 and 154.33) dm3 · mol−1 · s−1 at 20, 25, 30 and 35 °C, respectively. On the basis of the kinetic observations and evaluated activation parameters, plausible associative mechanisms are proposed for both interaction processes.

Solution studies of dinuclear polyamine-linked platinum-based antitumour complexes

The aquation profiles of two novel dinuclear polyamine-linked, platinum-based antitumour complexes [{trans-PtCl((15)NH(3))(2)}(2){μ-((15)NH(2)(CH(2))(6)(15)NH(2)(CH(2))(6)(15)NH(2))}](3+) (BBR3007, 1,1/t,t-6,6, 1) and [{trans-PtCl((15)NH(3))(2)}(2){μ-((15)NH(2)(CH(2))(6)(15)NH(2)(CH(2))(2)(15)NH(2)(CH(2))(6)(15)NH(2))}](4+) (BBR3610, 1,1/t,t-6,2,6, 1') have been probed using 2D [(1)H, (15)N] HSQC NMR spectroscopy. Reported herein are the rate constants for the hydrolysis of 1 and 1', as well as the acid dissociation constants of the coordinated aqua ligands in their aquated derivatives. The aquation and anation rate constants for the single step aquation model in 15 mM NaClO(4) (pH 5.4) at 298 K are, for 1, k(1) = 7.2 ± 0.1 ×10(-5) s(-1), k(-1) = 0.096 ± 0.002 M(-1) s(-1) and, for 1', k(1) = 4.0 ± 0.2 × 10(-5) s(-1), k(-1) = 1.4 ± 0.1 M(-1) s(-1). The effect of the linker backbone (Pt(tetra(m)mine vs. polyamine) was evaluated by comparison with previous data for the trinuclear complex [{trans-PtCl(NH(3))(2)}(2)(μ-trans-Pt(NH(3))(2){NH(2)(CH(2))(6)NH(2)}(2))](4+) (1,0,1/t,t,t or BBR3464). The pK(1) for 1,0,1/t,t,t (3.44) is closest to that of 1 (3.12), while the pronounced difference for 1' (4.54), means that 1' is the least aquated of the three complexes at equilibrium. pK(a) values of 5.92 were calculated for the aquated forms of both 1 and 1', which are 0.3 pK units higher than for either 1,0,1/t,t,t, or the dinuclear 1,1/t,t. The higher pK(a) values for both polyamine-linked compounds may be attributed to the formation of macrochelates between the central NH(2) groups and the {PtN(3)O} coordination sphere of the aquated species.

Determination of the kinetic profile of a dinuclear platinum anticancer complex in the presence of sulfate: introducing a new tool for the expedited analysis of 2D [(1)H,( 15)N] HSQC NMR spectra.

Two-dimensional (2D) [(1)H, (15)N] heteronuclear single-quantum coherence (HSQC) NMR experiments of the kinetics of aquation and sulfation of the dinuclear platinum anticancer complex [{trans-PtCl(NH(3))(2)}(2)(μ-NH(2)(CH(2))(6)NH(2))](2+) (1,1/t,t, 1) in 15 mM sulfate solution are reported using conditions (298 K, pH 5.4) identical to those previously used for other anionic systems (phosphate and acetate), allowing for a direct comparison. Sulfate is the fourth most abundant anion in human plasma. The rate constant for the aquation step (k(H)) is higher than that previously found in the presence of phosphate, but the anation rate constants are similar. The rate constant for sulfate displacement of the aqua ligand (k(L)) is approximately three times higher than that of phosphate, and a further major difference between these two anions is the very high k(-L) for loss of sulfate, suggesting that when formed in plasma the sulfato species will be substitution labile. We also introduce a novel (free) plug-in, '2D NMR analysis', developed for the expedited integration and analysis of 2D [(1)H, (15)N] HSQC NMR spectra. We have found that this plug-in significantly reduces the amount of time taken in the analysis of experiments with no loss to the quality of the data.

Mechanistic studies of substitution reaction of trans-(diaqua)-(N,N' -ethylene-bis-salicylamide)chromium(III) ion

The kinetics of substitution of trans-[Cr(Salm)(OH 2 ) 2 ]+ with some biologically important ligands (Nu - ) viz. glycine (Gly), p-aminobenzoic acid (Paba), glycyl-glycine (Gly-gly), L -histidine (His), L -isoleucine (Ile) has been studied spectrophotometrically over the range: 25 ≤ 5 t ≤ 45 °C, 1.8 ≤ pH ≤ 5.5, 0.01 ≤ [Nucleophile] ≤ 0.30, I = 0.5 mol dm -3 (KNO 3 ). Although there are two replaceable aqua ligands in the complex, only mono substitution occurred, which is evident from Job's curve. The kinetic studies also showed one aqua ligand substitution. Unlike trans-[Cr(Salen)(OH 2 ) 2 ] + , the rates of aqua ligand substitution were found slower indicating moderate labilization of axial aqua ligand. The reaction takes place via an outersphere association between the Cr III complex and various nucleophiles followed by transformation of the outer into inner sphere complexes by slow interchange. The anation rate constants (10 4 k an /s -1 ) at 25 °C were found to be 14.98 (Gly), 8.45 (Ile), 10.52 (Paba), 14.23 (His) and 13.78 (Gly-gly) and the corresponding ΔH # (kJ mol -1 ) values are 57.7 ± 1.6, 38.3 ± 1.3, 43.0 ± 0.8, 34.2 ± 2.7 and 39.7 ± 3.1, the ΔS # (JK -1 mol -1 ) values are -105 ± 5, -175 ± 5, -157 ± 3, -65.8 ± 9 and -166 ± 11. The higher value of the k an in comparison to k ex and highly negative values of activation entropy and variation of k an by changing nucleophiles, support I a mechanism for the substitution reactions.

Effects of geometric isomerism in dinuclear platinum antitumor complexes on aquation reactions in the presence of perchlorate, acetate and phosphate

The aquation and subsequent reactions of the dinuclear Pt antitumor complexes [{trans-PtCl(NH(3))(2)}(2)(mu-NH(2)(CH(2))(6)NH(2))](2+) (1,1/t,t) and [{cis-PtCl(NH(3))(2)}(2)(mu-NH(2)(CH(2))(6)NH(2))](2+) (1,1/c,c) in 15 mM perchlorate, acetate or phosphate solutions were followed at 298 K by [(1)H,(15)N] HSQC 2D NMR spectroscopy. Rate and equilibrium constants for the initial reversible aquation and the subsequent reversible reaction with phosphate or acetate are reported. The rate constant for the first aquation step is two-fold lower for 1,1/c,c than 1,1/t,t but the anation rate constants are similar so that the equilibrium lies further towards the chloro form for the 1,1/c,c compound. A pK (a) value of 6.01+/-0.03 was determined for the diaquated species [{cis-Pt(NH(3))(2)(H(2)O)}(2)(mu-NH(2)(CH(2))(6)NH(2))](4+) (1,1/c,c-3) which is 0.4 units higher than that of the 1,1/t,t compound. The rate constants for the binding of acetate and phosphate to 1,1/t,t are similar, but the rate constant for the reverse reaction is close to ten-fold higher in the case of phosphate so that equilibrium conditions are attained more rapidly (12 h compared with 64 h). On the other hand, for 1,1/c,c the rate constants for the forward and reverse reactions with acetate and phosphate are quite similar so that equilibrium conditions are reached very slowly (80-100 h) and a greater proportion of phosphate-bound species are present. The reduced lability of the bound phosphate for 1,1/c,c is attributed to the formation of a macrochelate phosphate-bridged species which was characterized by (31)P NMR and ESI-MS. The speciation profiles of 1,1/t,t and 1,1/c,c under physiological conditions are explored.

Influence of dT20 and [d(AT)10]2 on Cisplatin Hydrolysis Studied by Two‐Dimensional [1H,15N] HMQC NMR Spectroscopy

The influence of the presence of DNA on the kinetics of cisplatin (cis-[PtCl2(NH3)2]) aquation (replacement of Cl- by H2O) and anation (replacement of H2O by Cl-) involved in the hydrolysis of cisplatin have been determined by two-dimensional [1H,15N] HMQC NMR spectroscopy. Single-stranded dT20 and double-stranded [d(AT)10]2 oligonucleotides were used as DNA models, avoiding guanines which are known to react rapidly with aquated cisplatin forms. Reactions starting from cis-[PtCl2(15NH3)2], or from a stoichiometric mixture of cis-[Pt(15NH3)2(H2O)2]2+ and Cl- (all 0.5 mM Pt(II); in ionic strength, adjusted to 0.095 M or 0.011 M with NaClO4, pH between 3.0 and 4.0) were followed in an NMR tube in both the absence and presence of 0.7 mM dT20 or [d(AT)10]2. In the presence of dT20, we observed a slight and ionic-strength-independent decrease (15-20 %) of the first aquation rate constant, and a more significant decrease of the second anation rate constant. The latter was more important at low ionic strength, and can be explained by efficient condensation of cis-[Pt(15NH3)2(H2O)2]2+ on the surface of single-stranded DNA, in a region depleted of chloride anions. At low ionic strength, we observed an additional set of [1H,15N] HMQC spectral signals indicative of an asymmetric species of PtN2O2 coordination, and we assigned them to phosphate-bound monoadducts of cis-[Pt(15NH3)2(H2O)2]2+. Double-stranded [d(AT)10]2 slowed down the first aquation step also by approximately 15 %; however, we could not determine the influence on the second hydrolysis step because of a significant background reaction with cis-[Pt(NH3)2(H2O)2]2+.

Reversible hydrolysis of [PtCl(dien)] + and [PtCl(NH 3) 3] +. Determination of the rate constants using UV spectrophotometry

Hydrolysis of the platinum(II) complexes [PtCl(dien)] + and [PtCl(NH 3) 3] + was investigated in order to settle the apparent conflict between observations of a solvent-assisted pathway in chloride substitution reactions on the one hand, and the failure to detect chloride release on the other hand. Using UV spectrophotometry, we were able to follow the hydrolysis of both complexes, as well as the reverse anation reactions. The hydrolysis ( k H 2O ) and chloride anation ( k Cl) rate constants were determined in acidic aqueous solution (4.2 ≤ pH ≤ 5) at20 °C as k H 2 O = 6.5 × 10 −5 s −1; k Cl = 0.54 M −1 s −1 for [PtCl(dien)] +, and k H 2 O = 1.1 × 10 −5 s −1; k Cl = 0.14 M −1s −1 for [PtCl(NH 3) 3] +. It appears that the relatively fast anation, which limits the extent of hydrolysis, has prevented the hydrolysis from being detected in some of the previous investigations.

Hydrogen Carbonate Inhibited and Induced Substitution Reactions of Labile Square‐Planar Diethylenetriaminepalladium(II) Complexes

AbstractThe reaction of labile square‐planar diethylenetriaminepalladium(II) complexes with hydrogen carbonate ion was studied by stopped flow spectrophotometry as a function of [HCO−3] and [Cl−] and constant [Pd], ionic strength and temperature to investigate the substitution behaviour with a nucleophile capable of affecting the solution pH. The kinetic results significantly deviate from those for normal square‐planar substitution behaviour. The reaction comprises two consecutive equilibrations, which have complex kinetics compared to the simple two‐term rate law normally encountered, and which represent hydrogen carbonate inhibited and induced substitution reactions, respectively. The suggested mechanism and data treatment account for the observed deviating kinetics and provide alternative ways of obtaining formerly determined aquation and anation rate constants with comparable accuracy, even for complexes too labile to be monitored directly.

KINETICS AND MECHANISM OF THE REACTION OF AQUACHROMIUM(III) WITH L-ASCORBIC ACID IN AQUEOUS MEDIA

The kinetics of the reaction of l-ascorbic acid (H2A) with aquachromium(III) has been studied over the range 0.04 ≤ [H2A]T ≤ 0.16 mol dm−3, 3.5 ≤ pH ≤ 6.0, 25.0 ≤ t ≤ 50.0∘C, 0.03 ≤ I ≤ 1.0 mol dm−3 (KNO3) and 0% ≤ [MeOH] ≤ 20% (v/v). The reaction takes place via an outer-sphere association between Cr3+/CrOH2+ and H2A, followed by transformation of the outer- into an inner-sphere complex by slow interchange. The anation rate constants for Cr3+ and CrOH2+ species (k1 and k2) at 25∘C and I=1.0moldm−3 (KNO3) are 1.68×10−4 and 2.14×10−4s−1, respectively. ΔH‡ and ΔS‡ for the k1 and k2 paths are 27.0±2kJmol−1, −227±5J K−1mol−1 and 57.2± 4kJmol−1, −123±14JK−1mol−1, respectively. Anation of Cr(H2O)3+6 and Cr(H2O)5OH2+ follow an Ia path.

Steric effects on the substitution of pentaamine-chromium(III) and -rhodium(III) complexes. Anation reaction rate constants as an indicative of the dissociative shift of the mechanism on crowding the [M(RNH 2) 5H 2O] 3+ (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) complexes

The kinetics of the anation reactions of [M(RNH 2) 5H 2O] 3+ (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) with several ligands (H 3PO 4/H 2PO 4 −, H 3PO 3/H 2PO 3 −, CF 3COO −, Br −, Cl −, SCN −) have been studied at different temperatures and acidities at I = 1.0 M (LiClO 4. Results obtained for the anation rate constants and thermal activation parameters are compared with the previously published data for R = H, in order to establish the effects of the amine substituents in the reaction mechanism proposed for the substitution reactions of these complexes. The results obtained are interpreted on the basis of a mechanism where the bond formation process is more important in the substitution on M = Cr complexes than in that of the M = Rh complexes, as already pointed out for the published ΔΛ ≠ values for the water exchange on these systems. A simple Langford-Gray classification becomes inadequate to describe these situations where the increase of the steric demand of the amine substituents shifta the I a-I d classification to the I d side, although no dramatic changes in the reaction mechanism are found. It is concluded that a More O'Ferall ‘continuous’ type of approach to the mechanism classification of the substitution reactions is much more useful in this case.

Kinetics and mechanism of the displacement of aqua ligands from cis-diaqua-bis(biguanide)chromium(III) by pyridine-2-aldoxime in EtOH-H2O mixtures

Kinetic and mechanistic studies of the displacement of aqua ligands from cis-[Cr(BigH)2(H2O)2]3+ (BigH = H2NC(NH)CHC(NH)NH2) by pyridine-2-aldoxime (LH) in EtOH-H2O mixtures, in the 30–45° C range, obey the following rate law over the 3.5–6.0 pH range, $$\frac{{d[Cr(BigH)_2 (L)^{2 + } ]}}{{dt}} = \frac{{k_a K_E [Cr(BigH)_2 (H_2 O)_2^{3 + } ]_T [LH]}}{{1 + K_E [LH]}}$$ where KE is the ion pair equilibrium constant and ka is the rate constant for the interchange of the outer-sphere complex to inner-sphere complex. The anation rate constants are dependent upon pH but independent of ionic strength. Activation parameters (ΔH≠ and ΔS≠) have been evaluated from Eyring plots and a mechanism (Ia) involving outer-sphere association of reactants followed by associate interchange of this outer-sphere complex into product is proposed.

Kinetics and mechanism of anation reactions of [M(NH3)5(H2O)]3+ by H3PO n /H2PO n ? Systems (M = Cr III , n= 3; M = Co III , n= 2)

The kinetics of anation of [Cr(NH3)5(H2O)]3+ with H3PO3/H2PO3– and of [Co(NH3)5(H2O)]3+ with H3PO2/H2PO2– has been studied at different temperatures and at I= 1.0 mol dm–3(LiClO4). The results are interpreted in terms of an Ia(near the Ia/Id border) anation mechanism for the chromium complex and an Id one for the cobalt complex. Comparison of the anation rate constants with those of other anating systems and the same transition-metal complexes confirms this assignment. A comparison of all the anation reactions [M(NH3)5(H2O)]3++ H3POn/H2POn–(M = CoIII or CrIII; n= 2–4) is made, and two unexpected facts are examined: (a) for M = CrIII, first-order rate constants for the neutral (k1) and anionic (k2) species of the series H3POn/H2POn–(n= 2–4) are the same (k1=k2); and (b) for M = CoIII, even though the H2POn– species are always reactive, the conjugated species, H3POn, are not reactive, except for n= 2.

Solvent effects on the anation ofcis-diaquo-bis-ethylenediamine cobalt(III) by L-proline

The anation kinetics of the title complex were investigated spectrophotometrically in aqueous methanol, ethanol, ipropanol, t-butanol and dioxane, and the following rate law was established: $$d[Co(en)_2 (P)^{2 + } ]/dt = \frac{{k_a K_E [cis - Co(en)_2 (H_2 O)_2^{3 + } ]_T [P.H]}}{{1 + K_E [P.H]}}$$ An increase in proline concentration led to an increase in reaction rate with a limiting rate at higher ligand concentration due to ion-pair formation, where KE=the ion-pair equilibrium constant of the substrate proline complex. The nature of KE dependence on solvent composition is in good agreement with the ion-pair formation; an increase in the solvent organic component increasing KE. The anation rate constants (ka) are influenced by the solvent composition and decrease as the dielectric constant diminishes. In an isodielectric medium the anation rate remains constant. A good relationship between anation rate constant with Grunwald-Winstein coefficient (Y) is observed. Activation parameters (ΔH≠ and ΔS≠) have been calculated and compared with the isotopic water exchange process and a dissociative interchange (Id) mechanism involving ion-pair formation is proposed.

Kinetics and mechanism of anation ofcis-diaquo-bis-ethylenediamine cobalt(III) ion by L-serine in ethanol-water mixtures of varying dielectric constant

We propose the following rate law for the title anation reaction $$\begin{gathered} d[Co(en)_2 (Ser)^{2 + } ]/dt \hfill \\ = k_a K_E [cis{\text{ - Co(en)}}_{\text{2}} ({\text{H}}_{\text{2}} {\text{O)}}_{\text{2}}^{{\text{3 + }}} ]_{{\text{total}}} \hfill \\ \cdot [{\text{Ser}}{\text{.H]/\{ 1 + }}k_{\text{E}} [{\text{Ser}}{\text{.H]\} }} \hfill \\ \end{gathered}$$ within the 3.2 to 4.5 pH range, whereka is the anation rate constant, andKE the ion-pair equilibrium constant between the substrate complex ion [Co(en)2(H2O)2]3+ and incoming zwitterionic ligand serine (Ser.H). Reaction rates in different ethanol-water mixtures reach a limit at high ligand concentrations and theKE increases as the concentration of the organic component in the medium increases, whereas reverse is true forka. The reaction is catalysed by NO3− ion and the rate increases with the increase of pH within the quoted range. ΔH± and ΔS± values were obtained from an Eyring plot and are compared with data for the isotopic water exchange process. A mechanism involving ion-pair formation between two reacting species, followed by dissociative interchange of the outer-sphere complex with the product is proposed.

Kinetics and Mechanism of Anation of Pentaammineaquarhodium(III) Ion by Malonate in Weakly Acidic Media

Abstract Anation of pentaammineaquarhodium(III) ion by malonic acid, hydrogenmalonate, and malonate ions to form the hitherto unknown pentaamminemalonatorhodium(III) complex has been studied as a function of [H+] for the acidity range 1.4≤ pH ≤5.0, [total malonate], 0.3 mol dm−3 in the temperature range of 60 to 80 °C and at ionic strength 1.0 mol dm−3 adjusted with sodium perchlorate. Ion pairing is not significant between the complex and malonic acid but kinetically identifiable with hydrogenmalonate and malonate ions. Ion pair association constants for the hydrogenmalonate (Q1) and malonate ion (Q2) at 60 °C are 1.8 and 7.5 mol−1 dm3 respectively. The values of the rate constants at 60 °C for anation by malonic acid, hydrogenmalonate, and malonate ions are k0=3.04±0.2×10−5 mol−1 dm3 s−1, k1=1.73±0.2×10−4 s−1, and k2=3.46±0.4×10−4 s−1. The activation parameters, ΔH\eweq and ΔS\eweq corresponding to anation rate constants, k0, k1, and k2 are reported and the results discussed with reference to analogous reactions of Rh(III) and Co(III).

Kinetics and Mechanism of the Interaction of Iminodiacetate with cis-Tetraamminediaquacobalt(III) Ion in Acidic Media

Abstract Kinetics of the reaction of iminodiacetate (Hida−) with cis-tetraamminediaquacobalt(III) have been studied spectrophotometrically at pH 3.0–4.15, [iminodiacetate]T=0.03–0.4. mol dm−3, I=0.5 mol dm−3 (NaClO4), in the temperature range 30–50 °C. The reaction is governed by the rate law: rate={k−1+k1[iminodiacetate]T}[complex], where k−1 and k1 denote the reverse aquation and anation rate constants respectively. At 40 °C the k−1 values increase with pH and show a limiting tendency at pH≈4. The pH-anation rate profile indicates no reaction at pH<1.5 and in the range 3.5–3.9, k1 remains constant. The pH dependence on rate is expressed as k1=kAKAH/([H+]+KAH), where kA is the anation rate arising from the aqua complex ion and Hida− species and KAH, the dissociation constant for the iminodiacetic acid. The value of kA at 40 °C=(5±0.02)×10−5 mol−1 dm3 s−1. The activation parameters, ΔH\eweq and ΔS\eweq for the anation step are 170±2 kJ mol−1 and 239±3 JK−1mol−1. The reaction is believed to operate through the initial complexation at the carboxylate end followed by a fast intramolecular proton transfer with simultaneous ring closure to form a chelated product. The complexation probably takes place through a dissociative interchange (Id) mechanism.

Studies on the composition and kinetics of chromium(III)‐alanine system

AbstractKinetics of the complex formation of chromium(III) with alanine in aqueous medium has been studied at 45, 50, and 55°C, pH 3.3–4.4, and μ = 1 M (KNO3). Under pseudo first‐order conditions the observed rate constant (kobs) was found to follow the rate equation: Values of the rate parameters (kan, k, KIP, and K) were calculated. Activation parameters for anation rate constants, ΔH≠(kan) = 25 ± 1 kJ mol−1, ΔH≠(k) = 91 ± 3 kJ mol−1, and ΔS≠(kan) = −244 ± 3 JK−1 mol−1, ΔS≠(k) = −30 ± 10 JK−1 mol−1 are indicative of an (Ia) mechanism for kan and (Id) mechanism for k routes (‥substrate Cr(H2O) is involved in the k route whereas Cr(H2O)5OH2+ is involved in k′ route). Thermodynamic parameters for ion‐pair formation constants are found to be ΔH°(KIP) = 12 ± 1 kJ mol−1, ΔH°(K) = −13 ± 3 kJ mol−1 and ΔS°(KIP) = 47 ± 2 JK−1 mol−1, and ΔS°(K) = 20 ± 9 JK−1 mol−1.

KINETICS AND MECHANISM OF ANATION OFCIS-TETRAAMMINEDIAQUACOBALT(III) BY THIOCYANATE

Abstract The anation of cis-[Co(NH3)4(H2O)2]3+ with NCS− has been studied at 40–60°. The reaction proceeds in a stepwise manner leading to the formation of cis-[Co(NH3)4(NCS)2]+. Under pseudo-first-order conditions having [NCS−].=0.05–0.5 M. pH=2.00 and I=0.51 M (NaClO4), the following rate law is obeyed for each step, kobs=k0 + k[NCS−]. where k0 is the rate constant for the reverse (aquation) reaction and k is the second-order anation rate constant. The activation enthalpy and entropy values corresponding to k1, k2, k0 I and k0 II paths are δ H1 #=143 ± 7 kJ mol−1. δS# #=141 ± 11 J K−1 mol−1, δ H2 #=111 ± 6 kJ mol−1, δ S2 #=29 ± 8 J K−1 mol−1, δH0 1#=102 ± 7 kJ mol−1. δS0 #=3 ± 5 J K−1 mol−1, δH0 II#=91 ± 8 kJ mol−1 and δ S0 II#=-58 ± 7 J K−1 mol−1, respectively. The first-step reaction rate shows a strong inverse dependence on [H+] indicating that the base form of the complex. cis-[Co(NH3)4(OH)(H2O)]2+, is some 103 times more reactive than the conjugate acid form, cis-[Co(NH3)4(H2O)2]3+. Within experime...

KINETICS OF ANATION OF CIS-DIAQUO-BIS-(1,3-DIAMINOPROPANE) COBALT(III) BY OXALATE

Abstract The kinetics of anation of cis-diaquobis(1,3-diaminopropane) cobalt(III) by oxalic acid and bioxalate anion have been studied at 35°, 40° and 45°C, I=1.0 M(NaClO4). The observed rate law for the formation of Co(tmd)2C2O+ 4, is valid in the range of [Ox]T=0.03–0.50 M, pH=0.3–2.03. The anation rate constants, k1 and k2, for cis-{Co(tmd)2(OH2)3+ 2, H2C2O4) and cis-{Co(tmd)2(OH2)3+ 2, HC2O4-] ion pairs respectively are found to be virtually the same (k1=10.7 × 10−4 sec−1 and k2=11.0 × 10−4 sec−1 at 40°C). Temperature independent oxalic acid and bioxalate ion-pair association constants (Q1 and Q2) are 1.7(±0.3)M−1 and 11.5(±0.7)M−1, respectively. The activation enthalpy and entropy for k1 and k2 paths are 122.6 kJ mol−1 89.5 JK−1 mol−1 and 103.8 kJ mol−1, 29.3 JK−1 mol−1, respectively. The anation of cis-{Co(tmd)2(OH2)3+ 2, X} ion pairs (X=H2C2O4 or HC2O− 4) is believed to involve Id mechanism. The reaction is catalysed by NO− 3.