Kinetics, aquation

The branched isomer of trien, / ,/r,/r-triaminotriethylamine (tren 48e) can act as a tetradentate toward Rh111, or, as the trihydrochloric acid salt [(tren3H)7+3Cl" ], it can act as a trivalent cation for the precipitation of [RhCl3X3]3 (X = Cl, Br, I) and [RhBr6]3".771 First reported by Basolo,619 the aquation kinetics of [Rh(tren)Cl2]+ were later studied by Madan.772 Ligand field photolysis of [Rh(tren)Cl2]+ also causes chloride loss, but the photo and thermally produced [Rh(tren)Cl(H20)]2+... 

Ligand substitution at frans-[Ir(X)2(en)2]+ occurs without stereochemical change, and with substitution rates independent of the concentration and nature of the incoming ligand and the ionic strength. The substitution mechanism involves an aqua intermediate, as illustrated in Scheme 10.231 Aquation kinetics of complexes such as fra s-[Ir(X)2(en)2]+ were not directly accessible, the equilibrium in Scheme 10 lying to the left. Table 2 shows various ligand substitution rates and activation parameters for /ranj-[Ir(X)(en)2(L)]+. The base hydrolysis of cis- and mms--[Ir(Cl)2(en)2]+ also proceeds with stereoretention. 

The aquation kinetics of [Co(NH3)5Cl] and [Co(NH3)5Br] have been studied using acetone-water solvent over a wide range of solvent compositions and temperatures in an attempt to correlate the rate constant with the dielectric constant of the medium.The variation of thermodynamic properties of the activated complex with the mole fraction of acetone reveals the existence of specific solvation. 

The aquation of [Co(NH3)5Cl] has been studied in ethylene glycol-water mixtures over the temperature range 35 to 55 The effects of solvent composition on fcaq and the activation parameters are discussed in detail. The effects of ion pairing on the aquation kinetics of [Co(NH3)5Br] in the presence of maleate and phthalate anions has been studied at a series of temperatures. The rate-accelerating effects of the ions are attributed to the formation of more labile ion pairs. The rate and association constants of the ion pairs have been calculated. 

The aquation of complexes of ethylenediamine and its derivatives is often thought to proceed through intermediates containing unidentate ethylenediamine. To support such hypotheses, several chromium(m) complexes containing unidentate ethylenediamine have been isolated, and their aquation kinetics studied. The preparation of the first cobalt(m) complex containing unidentate ethylenediamine has now been reported. There is kinetic evidence for the formation of an intermediate [Co(tren)(OH2)-(COsH)] +, containing unidentate — COsH , in the aquation of [Co(tren) (COs)]+ in strongly acidic solutions. ... 

Effects of Non-leaving Ligands.— The usual way of assessing the effect of a unidentate non-leaving ligand L on reactivity of cobalt(m>-amine-halide complexes is to investigate aquation kinetics of compounds [Co(en)3-LC1] +. Recently described examples include complexes of cij-geometry,... 

Unfortunately the thallium(iii)-catalysed aquation kinetics have only been reported qualitatively (J. J. Conocchioli, G. H. Nancollas, and N. Sutin, Proc. Chem. Soc., 1964, 113 N. Sutin, Ann. Rev. Phys. Chem., 1966,17, 119). It is not therefore possible to compare the relative effectiveness of thallium(iii) and mercury(ii) as catalysts here with their effectiveness in aquation of cobalt(ui)-chloride complexes (c/., e.g., ref. 68 of this chapter). 

Complexes whose aquation kinetics conform to this pattern include [Cr(NH3)5(N3)] +, [Cr(NHs)6(03CCCl3)] +, and [CrCOH ) -... 

The aquation kinetics of the chloropentaamminecobalt(III) ion in water-ethanol mixtures has been studied. The rate constants correlate well with the Grunwald-Winstein Y parameter and with the dielectric constant of the medium. The data supports a D mechanism for the reaction. The loss of chloride from the complexes cw-[Co(en)2(NH2CH2CH20H)Cl] and cw-[Co(en)2(NH2(CH2)3 0H)Cl] has been studied in aqueous ethyleneglycol at 40-65 °C in acidic media and at 20-35 °C in basic media.The rate constants decreased linearly with the increasing mole fraction of the cosolvent. The loss of chloride resulted in the formation of the chelated amino-alcohols as the main product. The observed solvent isotope effect (A h2oAd2o) = 112 at 50 °C, [HCIO4] =0.01 moldm for chloride release is lower than the value reported for the aquation of the cw-[Co(en)2(alkylamine)Cl] complexes (1.38-1.44). This result may indicate the lack of direct solvent intervention in the act of substitution at the cobalt(III) center, as expected for a true intramolecular reaction. 

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