Effect of solvent and structure on the kinetics and mechanism of the aquation of bromopentaamine cobalt(III) complex in binary aqueous mixtures

Abstract

The kinetics of aquation of bromopentaamine cobalt(III) complex have been investigated spectrophotometrically in aqueous-organic solvent media using acetonitrile, urea, and dimethyl sulfoxide as co-solvents at 45 ≤ T ( • C) ≤ 65. The logarithms of rate constant of the aquation reaction vary nonlinearly with the reciprocal of the dielectric constant for all cosolvent mixtures, indicating a specific solute-solvent interaction. Also, the rate constants are correlated with the total number of moles of water and the organic solvents. However, the solvent effects on the solvation components of the enthalpy of activation, H ‡ , and the entropy of activation, S ‡ , have been studied. Analysis of the solvent effect confirmed a common I d mechanism for the aquation of the cobalt(III) complex. C

Spectrometric study of solute–solvent interactions for the solvolysis of chloropentaamine cobalt(III) complex in binary aqueous mixtures

“…Thus, most of these values are nearer to unity and others are different from unity, indicating that the internal structure of the media studied suffered from serious changes on addition of the organic solvent component. The same behavior was also observed for the solvolysis of the [Co(NH 3 ) 5 Br] 2+ cationic complex [7] and by Tommila et al [16]. They indicated that the hydrogen bond between organic solvent molecules and water is stronger than that between two water molecules.…”

“…It was found that the values of the rate constant for the solvolysis of [Co(NH 3 ) 5 Br] 2+ [7] were greater than that of [Co(NH 3 ) 5 Cl] 2+ under investigation as shown in Table I. These results can be attributed to the greater size of bromide ion.…”

“…In general, the rate constants for this complex decrease smoothly as the mole fraction of the cosolvent increases. The same behavior was observed for [Co(NH 3 ) 5 Br] 2+ with the same organic cosolvent and compositions [7]. The effect of water concentration in aqueous organic solvent mixtures was investigated by using the general rate equation (1):…”

“…Therefore, rates of chemical reactions are affected by the nature of the solvent used and by the nature and concentration of ionic species present in the reaction mixture. A group of substitution reactions that have been extensively studied are those involving solvolysis of pentaamine cobalt(III) complexes [7][8][9]. For many reactions, there would appear to be a balance between S N1 and S N2 character depending on the nature of the incoming and outgoing groups, their size, and nucleophilic characters.…”

Studies on ion-pairing effects of the kinetics of aquation of bromopentaammine cobalt(III) complex in malonate media

“…Also Log kip was plotted against reciprocal of the dielectric constant D at 25 o C, where the dielectric constant values of different compositions are obtained from Akerlof data [18]. As shown in Figure(3), the plot was found to be non linear in accordance with the general observations found in the aquation of a large number of other cobalt(III) complexes [19][20] in water-cosolvent mixtures, which led to a conclusion, that the contribution of the non electrostatic part of solvent effect, overcomes the electrostatic component part. Also, these parameter measures macroscopic properties, while specific solute-solvent interactions occur on a microscopic scale are completely neglected.…”