Metal ion catalysed aquation of carboxylatoamine cobalt(III) complexes. Kinetics of copper(II) catalysed aquation ofcis(diformato)bis(ethylenediamine)cobalt(III) ion

Summary The aquation ofcis-[(en)₂Co(CO₂H)₂]⁺ tocis-[(en)₂Co(OH₂)(CO₂H)]²⁺ is catalysed by Cu²⁺ and the rate equation, –d[complex]_t/dt=(k_Cu[Cu²⁺]+k_H [H⁺]) [complex)_T is valid at [Cu²⁺]_T=0.01–0.1, I=0.5 and [HClO₄]=0.005 mol dm^–3. The rate measurements are reported at 30, 35, 40 and 45°C and the rate and activation parameters for the Cu²⁺ and H⁺-catalysed paths are: k_H(35°C)=(2.44±0.09)×10^–2 dm³ mol^–1 s^–1, H=83±13 kJ mol^–1, S=–8±42 JK^–1 mol^–1, k _Cu (35°C)=(3.30±0.09)×10^–3 dm³ mol^–1 s^–1, H=73.2±6.1 kJ mol^–1, S=–55±20 JK^–1 mol^–1. The formate-bridged innersphere binuclear complex,cis-[(en)₂Co{(O₂CH)₂Cu}]³⁺ may be involved as the catalytically active intermediate in the copper(II)-catalysed path, just as the corresponding H⁺-bridged species presumed to be present in the acidcatalysed path.     

Mechanism of the S-->N isomerization and aquation of the thiocyanato pentaammine cobalt(III) ion

All of the stationary points on the potential energy surface of the S-->N isomerization and aquation of the Co(NH3)5SCN2+ ion have been investigated with ab initio quantum chemical methods. Also the corresponding anations of the Co(NH3)5OH2(3+) ion by the N and S ends of SCN- and the substitution of thiocyanate via the D mechanism have been studied. All calculations have been performed by taking into account hydration. The most favorable reaction of Co(NH3)5SCN2+ is the isomerization. It is concerted, follows the I or Id mechanism, depending on the applied criteria, and proceeds via a T-shaped transition state. The aquations of Co(NH3)5SCN2+ and Co-(NH3)5NCS2+ and the corresponding inverse reactions, the anations, all proceed via the Id mechanism. The activation energies, calculated for the isomerization and aquation, agree with experiment, and so does the difference of the activation energies for the anations by the two donors of SCN-. This energy difference reflects the disparate nucleophilicities of the N and S ends of SCN- and shows that bond making in the transition state is significant for the Id mechanism. Isomerization and aquation are two parallel reactions which proceed via two disparate transition states. The computed activation energy for the SCN- substitution via the D mechanism is the highest, and therefore, this pathway is unlikely to operate for the isomerization and aquation of Co(NH3)5SCN2+. The S-->N isomerization and the SCN- substitution via the D mechanism were furthermore computed for the free ions in the gas phase: the isomerization would require a higher activation energy and follow the Ia mechanism. The activation energy for the SCN- substitution via the D mechanism would be very high, because of the large electrostatic work which is required for the removal of an anion from a (formally) 3+ charged cation.     

Kinetics and mechanism of base hydrolysis of cis-(ammine) bis (ethylenediamine) (substituted salicylato) cobalt(III) complexes

The kinetics of base hydrolysis of cis-(ammine) bis (ethylenediamine) (substituted salicylato) cobalt(III) complexes, [Co{CO₂C₆H₃(X)(OH)}(NH₃)(en)₂]²⁺ (X = H, 5-SO₃⁻, 5-Br, 5-NO₂ and 3-NO₂), have been investigated in aqueous medium of I = 1.0 M. The phenate species, [Co{CO₂C₆H₃(X)(0)}(NH₃)(en)₂]⁺, have been found to undergo base hydrolysis via two path ways, i.e. one zero order in [OH⁻] (k₁ path) and another first order in [OH⁻] (k₂ path). The alkali independent rate constant (k₁) increases with the basicity of the phenate group and the plot of log k₁ against pK_OH of the complexes is a straight line with a positive gradient of 0.98±0.03. In contrast to this the plot of log k₂ against pK_OH has a gradient of −0.15±0.02. The latter correlation indicates that the electron withdrawing substituents in the salicylate moiety enhance the rate of base hydrolysis in the alkali dependent path. The reaction is not subject to imidazole catalysis in both k₁ and k₂ paths. Substantially high positive values of ΔS^≠ for both the paths are observed. Several mechanistic possibilities have been considered. SN₁CB mechanism involving the rate limiting CoO bond fission in the triagonal bipyramidal transition state appears to be best suited for both the paths. For the alkali independent path the unbound phenate group is suggested to generate the reactive conjugate base by abstracting the NH proton from the coordinated amine.     

Kinetics and mechanism of hydrolysis ofcis-bis(ethylenediamine)imidazole(salicylato)cobalt(III) ion

Summary The kinetics of aquation and base hydrolysis reactions ofcis-[(en)₂Co(imH)O₂CC₆H₄OH-o-o]²⁺ (imH = imidazole) have been investigated in a medium of 1.0 M ionic strength, In the 0,1–1,0 M [H⁺] range (60–70°) aquation proceedsvia spontaneous and acid catalysed paths . In the 0,05–1.0 M [OH^–] range (30–40°), the complex exists predominantly as the bis-deprotonated species,cis-[(en)₂Co(im)O₂CC₆H₄O-o], and the pseudo-first-order rate constant fits the relationship k_obs = k_b + k_b° [OH^–] satisfactorily. The labilizing action of coordinated imidazolate anion(im^–) on the cobalt(III)-bound salicylate is 10³ times stronger than that of imidazole. The mechanism is essentially I_d in the aquation paths and SN₁cb (Co-O bond fission) in the alkali independent and dependent paths respectively.     

Kinetics of aquation ofcis- andtrans-chloro-nitrobis-(ethylene-diamine)cobalt(III) ion in binary aqueous mixtures

Kinetics of aquation ofcis- andtrans-[Co(en)₂NO₂Cl]⁺ were investigated in aqueous mixtures of MeOH, EtOH,i-PrOH andt-BuOH at 298.1 K. The values of transfer functions corresponding to the transfer of reactant and activated complex from H₂O to the solvent mixtures were calculated from kinetic measurements and from solubilities of the complex salt. The results of the analysis of solvent effect are discussed in terms of D and I_d mechanisms. TMC 2676     

Kinetics of acid- and iron(III)-catalysed aquation of cis-(salicylato)-(methyl/ethylamine) bis(ethylenediamine)cobalt(III)

Summary The kinetics of spontaneous, acid- and Fe^III-catalysed aquation of cis-[Co(en)₂(RNH₂)(SalH)]²⁺ complexes (R = Me, Et; SalH = C₆H₄(OH)CO _inf2 ^sup- ) were studied in acid perchlorate medium, I = 1.5 mol dm^–3 (NaClO₄) at 70–80°C. The Fe^III-catalysed aquation proceeds via formation of a binuclear species, the evidence of which follows from aquation, complexation and equilibrium studies. The spontaneous aquation rate shows steric acceleration with the increase of the nonlabile amine chain length, while that of acid- and Fe^III-catalysed aquation shows the opposite trend. An attempt is made to explain the discrepancy in the rate on the basis of solvent cosphere effects.     

Kinetics of aquation of the trans-dichlorobis (N-methylethylenediamine) cobalt(III) ion in a water-methanol media

Summary The trans-[Co(meen)₂Cl₂]Cl complex was prepared and characterized by elemental analysis, and u.v.-vis. and i.r. spectroscopies. The kinetics of the primary aquation of trans-[Co(meen)₂Cl₂]⁺ in H₂O and H₂O-MeOH have been investigated over a wide range of solvent compositions and temperatures (45–60 °C). Plots of rate constants (log k) versus the reciprocal of the dielectric constant of the medium (D _infs ^sup−1 ) and Grunwald-Winstein values of the solvent (Y) were non-linear. The variation of enthalpies (ΔH ^*) and entropies (ΔS ^*) of activation with solvent composition have been determined. Plots of ΔH ^* or ΔS ^* versus the mole fraction of the solvent exhibit a maximum at x ₂ ca. 0.1 and a minimum of x ₂ ca. 0.3; a linear plot of ΔH ^* versus ΔS ^* is obtained. Furthermore, the cycle relating the free energy of activation in H₂O to that in H₂O-MeOH shows that changes in the solvent structure in H₂O-MeOH mixtures generally stabilize the five-coordinate cation in the transition state, more than the cation in the initial state as the mole fraction of MeOH increases. The results are discussed and compared with other related systems.     

Kinetics of aquation of cis-aquabromo-bis(ethylenediamine) complexes of cobalt(III) and chromium(III) in binary aqueous mixtures

Summary The kinetics of aquation of cis-[Co(en)₂(H₂O)Br]²⁺ and cis-[Cr(en)₂(H₂O)Br]²⁺ (en = ethylenediamine) were investigated in aqueous mixtures of MeOH, EtOH, i-PrOH and t-BuOH. The values of transfer functions corresponding to the transfer of reactants and activated complex from water to the solvent mixtures were evaluated from kinetic measurements and from solubilities of the complex salt. Analysis of the solvent effect confirmed a common I_d mechanism for the aquation of the Co^III and Cr^III complexes.     

Mechanisms of the aquation of tris-(substituted) malonatochromate(III) complexes

Summary Investigations on the aquation of tris-(substituted)malonatochromate(III)ion, between 25°–40° in the presence of acid (I=1.0M) show that the mono- and dimethyl substituted complexes aquate by a two term rate law, only one of which is acid dependent. All other complexes aquate by a one term rate law which is first-order with respect to acid concentration; the unsubstituted malonato complex behaves in this way. Aquation of the first two complexes is dissociative, while for others appreciable associative character has been observed.     

Salt and medium effects in the aquation ofcis-amminebromobis(ethylenediamine)cobalt(III) ion

Summary The aquation kinetics ofcis-[Co(NH₃)Br(en)₂]²⁺ were studied in aqueous and mixed aqueous-nonaqueous solvents in the presence of KBr, KCl, LiCl, NaNO₃ and NaClO₄ as supporting electrolytes. It was found that the association between the complex ion and Cl^–, Br^– and NO ₃ ^– ions takes place. The association constants, K_IP, were evaluated from kinetic measurements. The kinetic parameters in the studied media are related to the influence of the cosolvents on the solvation spheres of the reactants and activated complex.     

The kinetics of aquation of cis-ammine-bromobis(ethylenediamine) cobalt(III) ion in water-organic mixtures

The aquation kinetics of cis-[CoNH₃Br(en)₂]²⁺ were investigated in aqueous mixtures of ethyl alcohol, isopropyl alcohol, t-butylalcohol and acetonitrile. The rate constants, activation enthalpies and entropies were determined. The interpretation of the experimental data, based on electrostatic theory, was possible only when methyl alcohol and ethyl alcohol were used as co-solvents. It was shown that in the mechanism of reaction studied non-electrostatic interactions play an important role.     

Reactions of coordinated ligands,Part III(1). Kinetics of lodination of malonate and pyruvate in malonato(pentaammine)-cobalt(III), malonato-bis-(ethylenediamine)cobalt(III) and pyruvato(pentaammine)cobalt(III) ions

Summary The kinetics of iodination of malonate and pyruvate in the title complexes are reported at 35.0 °C and I=0.3 M. The reaction is first order in substrate and zeroth order in [I₂]. This result is commensurate with rate determining enolisation of the active methylene and methyl groups of the malonate and pyruvate respectively. The reaction is catalysed by H₂O, OH^– and by the buffer anions used. The rate data suggest that the malonate methylene group in the [Co(en)₂-O₂CCH₂CO₂]²⁺ chelate is considerably more active towards electrophilic substitution than is the case in [Co(NH₃)₅O₂CCH₂CO₂]²⁺.     

Kinetics and mechanisms of the reactions of unsymmetrical chelate complexes. Acid-catalyzed aquation of glycinatobis-(biguanide)cobalt(III) perchlorate

The presence of the unsymmetrical glycinato chelate in this complex, makes the positions of the two biguanides non-equivalent. Thus the acid-catalyzed dissociation of one or other of them, gives two different isomeric (α- and β-) diaquo- products. Up to ∼50°C, the dissociation of only one biguanide takes place with the formation of the α- diaquo product. The α- diaquo form isomerizes to the β- form at higher temperature. Direct formation of the β- form does not take place at any stage of the reaction. Aquation of the bis(biguanide) complex follows a dissociative pathway up to ∼50°C, above which it becomes a complicated one. Isomerization of the α- product takes place intramolecularly by two paths, one dependent and the other independent of acid concentration. Both the isomerization paths involve “bond-rupture” processes.     

Kinetics of aquation of cis- and trans-chloro-nitrobis(ethylenediamine) cobalt(III) ion in aqueous mixtures of ethylene carbonate and propylene carbonate

Summary The kinetics of aquation of cis- and trans-[Co(en)₂-NO ₂Cl] ⁺ were investigated in quasi-isodielectric aqueous mixtures of ethylene carbonate (EC) and propylene carbonate (PC) over a range of temperatures. Transfer Gibbs energies of reactant and transition state from H₂O to aqueous mixtures of EC were evaluated from kinetic measurements and the solubilities of the complex salt. An analysis of the solvent effect on the solvation of the initial (IS) and transition (TS) state reveals that their stabilization by non-aqueous cosolvent is due to predominant ion-dipole and dispersion interactions.     

Synthesis and mechanistic studies on the aquation of selenitoaquobis(diamine)cobalt(III) complexes

Summary The kinetics of the aquation of four selenitoaquobisdiamine)cobalt(III) complexes to their respective diaquabis-(diamine)complexes (diamine=ethylenediamine en, propylenediamine pn, dimethylethylenediamine me₂en and trimethylenediamine tmd) have been carried out conductimetrically in the 25–45 °C range. All reactions exhibit simple first order kinetics, and the rates increase with increasing temperature. In aqueous solution, the complex species exist in equilibrium with their respective hydroxo species, but only the hydroxo species are involved in the aquation process. This result is also reflected in the linear Arrhenius plot. The rates are higher in a 10% alcoholic solution than in water, but then decrease with increase in the alcohol content. A Grunwald-Winstein plot of rates in methanol yields slopes of 0.18 ± 0.04, 0.27±0.03, 0.43±0.03 and 0.34±0.02 for the four amines respectively. A dissociation mechanism is proposed for the aquation of all these complexes.     

Base hydrolysis ofcis-(methyl/ethylamine)bis(ethylenediamine) (salicylato)cobalt(III) complexes

The kinetics of the base hydrolysis ofcis-[Co(en)₂(RNH₂)-(SalH)]²⁺ (R=Me or Et; SalH=HOC₆H₄CO ₂ ⁻ ) were investigated in aqueous ClO ₄ ⁻ in the 0.004–0.450 mol dm⁻³ [OH⁻] range, I=0.50 mol dm⁻³ at 30–40°C. The phenoxide species is hydrolysed via [OH⁻]-independent and [OH⁻]-dependent paths, the latter being first order in [OH⁻]. The high rate of alkali-independent hydrolysis of the phenoxide species is associated with high ΔH^≠ and ΔS^≠ values, in keeping with the SNICB mechanism involving an amido conjugate base generated by the phenoxide-assisted NH-deprotonation of the coordinated amine. The [OH⁻]-dependent path also involves the conventional SN₁ CB mechanism. The rate constant, k₁, for the SNICB path exhibits a steric acceleration with the increasing size of the non-labile alkylamine, whereas the rate constant, k₂, for the SN₁CB path shows a reverse trend. TMC 2578