Mechanisms of reactions of metal complexes: Some recent developments

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, wherek
a is the anation rate constant, andK
E 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 theK
E increases as the concentration of the organic component in the medium increases, whereas reverse is true fork
a. The reaction is catalysed by NO
 3
 −
 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 ofcis-diaquo-bis-ethylenediamine cobalt(III) ion by L-serine in ethanol-water mixtures of varying dielectric constant

The kinetics of base hydrolysis of [Co(NH3)5Cl]2+ and [Co(NH3)5SO4]+ were investigated in aqueous mixtures of MeOH, EtOH,i-PrOH andt-BuOH at 298.1K. The values of transfer functions of the transition state corresponding to the transfer from water to the mixed media were calculated from solubilities of the complex salt and from kinetic measurements. The results of the analysis of the medium effect on the solvation of the initial and transition state are discussed in terms of the SNl CB and Id mechanisms.

Base hydrolysis of pentaammine complexes of cobalt(III) in aqueous-organic mixed solvents

Kinetic studies in water-ethanol solution on the replacement of the aqua-ligands in [Cr(H2O)5(OH)]2+ by 2,2′-bipyridine (bipy) and 1, 10-phenanthroline (phen) have been made spectrophotometrically. In both the cases the reaction proceeds by a path which is first order with respect to the base. The reaction is catalysed by NO3− and the rate increases with the increase of pH. We propose the rate law

$$k_{obs} = k_1 k_2 \left[ L \right]/\left( {k_{---1} + k_2 \left[ L \right]} \right)$$

where k1 is the water dissociation rate of the substrate complex, k−1 is the aquation and k2 is the ligand capturing rate of the pentacoordinate intermediate [Cr(H2O)4(OH)]2+. Experimental results are consistent with a dissociative mechanism. Variation of dielectric constant of the medium was used to verify the mechanistic conclusion in the reaction with bipy.

Mechanistic studies on the reactions of pentaaquahydroxochromium(III) ion with 2,2'-bipyridine and 1,10-phenanthroline

Kinetic and mechanistic studies on the anation of cis-[Ru(tap)2(H2O)2]2+ (where tap = 2-(m-tolylazo)pyridine) by pyridine-2-aldoxime (LH) have been made spectrophotometrically at different temperatures (35–50° C) in aqueous medium and various EtOH-H2O mixtures. The following rate law has been established at pH 5.6: kobs = k1k2[LH]/(k−1 + k2[LH]) where k1 is the H2O dissociation rate constant of the substrate complex and k-1 and k2 are the aquation and ligand capturing rate constants of the pentacoordinate intermediate, [Ru(tap)2(H2O)]2+. The rate constants are independent of ionic strength. The reaction rate increases with increasing pH. Activation parameters (ΔH‡ and ΔS‡) have been calculated for media of four different dielectric constants and compared with other substitution reactions in aqueous medium. A dissociative mechanism is proposed.

Kinetics of substitution of cis-diaqua-bis[2-(m-tolylazo)pyridine] ruthenium(II) ion with pyridine-2-aldoxime in EtOH-H2O mixtures

In the base hydrolysis of the title complexes in aqueous-organic solvent media (water-ethanol and water-acetone) to the corresponding hydroxo-complexes the observed rate law is in accord with the expression: The observed K values (3.5 to 26 M−1 at 25°) and the nature of their solvent dependence suggest the formation of an ion-pair in a pre-equilibrium involving the substrate complex and OH− ion, and not of a conjugate base. Rate constants (k) for the transformation of the ion-pair to the product increase with increased proportions of the organic component in the solvent. The nature of the dependence of k on different solvent parameters indicates significant associative character with OH− ion acting as the attacking nucleophile. Activation enthalpy, ΔH≠, decreases with ethanol concentration; this along with the observed large positive slopes of the log k vs 1/D plots rule out the possibility of H2O acting as the attacking nucleophile. ΔH≠ and ΔS≠ corresponding to k in different solvent composition...

R. N. Banerjee

Papers

Base hydrolysis of iso-thiocyanatopentaammine complexes of cobalt(iii), chromium(iii) and rhodium(iii) in aqueous-organic mixed solvents