Ibrahim M. Sidahmed

The solvolysis oftrans-[Co(4-Mepy)4Cl2]ClO4 in water-methanol mixtures

SummaryThe solvolysis of thetrans-[Co(4-Mepy)4Cl2]ClO4 complex was studied in 0 to 70% v/v H2O: MeOH mixtures at 40, 45, 50 and 55 °C. The high negative ΔS* values found for the complex cation under investigation, relative to that oftrans- [Co(py)4Cl2]+ reported in the literature, were attributed to the substituent methyl groups. The free energies of transfer of both the ground and the transition states were calculated from which the dominant effect of the solvent on the transition state is apparent.

Kinetics of the solvolysis of [Co(Rpy)4Cl4]+ with R = 4-t-Bu, 3-Me or 3-Et in mixtures of urea with water

SummaryThe kinetics of the solvolysis of complex ions trans-[Co(Rpy)4Cl2]+, with R = 4-t-Bu, 3-Me and 3-Et, have been investigated in mixtures formed by adding urea to water, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of [Co(Rpy)4Cl2]+ in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the Co3+ cation in the transition state is more stable than the Co3+ cation in the initial state in the water + urea mixtures.

Solvent effects on the initial and transition states upon solvolysis oftrans-dichlorotetra(4-ethylpyridine) cobalt(III) perchlorate in water +t-butanol mixtures

SummaryThe solvolysis of the title complex was investigated in water and in water +t-butanol mixtures in the 40 to 55°C range. The activation thermodynamic parameters were calculated and the extrema observed for ΔH† and ΔS† were compared with data obtained from the measured physical properties of the same solvent mixtures. The ethyl substituent in the pyridine rings manifests its effect on the activation entropy values and on the relatively low activation energies compared with the respective values obtained previously from the solvolysis oftrans-[Co(4-MePy)4Cl2]+ in the same media. The free energies of transfer of the cation were calculated in the ground and transition states. In the ground state, the ethyl and methyl substituents in the pyridine rings provide the complex cation with nearly equal stabilities. However, the effect of solvent on the cation stability is more pronounced in the transition state for the ethyl substituent.

Solubility of dichlorotetraalkylpyridinecobalt III hexachlororhenate(IV) in water + methanol mixtures: Free energies of transfer of the complex cation from water into the mixtures

The solubility of salts [Co(3Rpy)4Cl2]2]ReCl6] has been determined in water + methanol mixtures. By comparing these with the solubilities of the salt Cs2ReCl6 and using calculated activity coefficients for the ions in the water+methanol mixtures, values for {ΔGto(Co(3Rpy)4Cl2+)−ΔGto(Cs+)} can be determined where ΔGto is the standard Gibbs free energy of transfer from water to an aqueous mixture. ΔGto(Cs+) from the solvent sorting scale and from the TPTB scale are then used to calculate ΔGto(Co(3Rpy)4Cl2+). These two sets of values for ΔGto(Co(3Rpy)4Cl2+) on the differing scales are then inserted into a free energy cycle applied to the bond extension Co(3Rpy)4Cl2+ (initial state)→Co(3Rpy)4Cl2++Cl− (transition state) for the solvolysis in water and in water + methanol mixtures to produce values for ΔGto(Co(3Rpy)4Cl2+) using both scales. Data for the solubilites of [Copy4Cl2]2[ReCl6] and [Co(4Rpy)4Cl2]2[ReCl6] have been re-calculated to compare free energies of transfer for these complex cations with those specified above.

Solubilities in aqueous mixtures oft-butanol andtrans-dichlorotetra(3-alkylpyridine) cobalt(III) hexachlororhenate(IV): Gibbs energies of transfer of the complex cations

The solubilities of the hexachlororhenate(IV) salts of the complex cations trans-[Co(3Mepy)4Cl2]+ and trans-[Co(3Etpy)4Cl2]+ have been determined in water+t-butyl alcohol mixtures. By reference to the solubilities of Cs2ReCl6 and the Gibbs energies of transfer of Cs+ from water into water+t-butyl alcohol mixtures, ΔGto(Cs+), ΔGto[Co(3Mepy)4Cl2+] and ΔGto [Co(3Etpy)4Cl2+] are calculated. These latter values, when introduced into the equation for a free energy cycle applied to the process of the initial state going to the transition state for the solvolyses of these two cations, produces values for ΔGto[Co(3Mepy)4Cl2+*] and ΔGto[Co(3Etpy)4Cl2+*] for the Co3+ cations in the transition state. These values are compared with (ΔGto(i) for i=[Co(Rpy)4Cl2]+, [Co(Rpy)4Cl]2+*, [Coen2XCl]+ and [Coen2X]2+* to investigate the influence of the hydrophobicity of the surface of the complex on its stability in the mixtures. ΔGto(i) (solvent sorting) are compared with ΔGto(i) (TATB).

The influence of solvent structure on the solvolysis of trans-dichlorotetra(4-ethylpyridine) cobalt(III) perchlorate

SummaryThe solvolysis oftrans-[Co(4-Etpy)4Cl2]ClO4, was followed spectrophotometrically in water/isopropanol at different temperatures. The activation energy varied nonlinearly with the mole fraction of the co-solvent,χ2. The plot of logk versus Ds−1 was also non-linear. These features were attributed to the differential solvation of the initial and transition states. On plotting ΔH† versus ΔS†, the points fall very close to straight line. The isokinetic temperature was found to be 334K, indicating that the solvolysis reaction is controlled by ΔS† and not ΔH†. The change in ΔH† and ΔS† with the mole fraction of the cosolvent shows extrema at the composition range where changes in solvent structure occur. The influence of the solvent structure on the complex ion in the transition state dominates over that in the initial state, where −ΔGt0[Co(4-Etpy)4Cl]2+>−ΔGt0[Co(4-Etpy)4Cl2]+.

The solvolysis oftrans-[Co(4-Mepy)4Cl2]ClO4 in water: acetonitrile and water: dioxan mixtures

SummaryThe solvolysis of the title complex is slower in 0 to 70% v/v aqueous dioxan than in aqueous acetonitrile at 40, 45, 50 and 55° C, a fact that is attributed to the higher basicity and ionizing power of the latter solvent mixtures. The observed non-linearity for logk (solvolysis) versus 1/D plots was interpreted in terms of the relative stability of ground and transition state species. The calculated Gibbs free energy for transfer of the species in the transition state is consistent with the extreme stretching of the chlorinecobalt bond, a result which confirms the interchange dissociative mechanismId for the complex under study.