Kinetics of the solvolysis of [Co(3Rpy)4Cl2]+ ions (R = Me or Et) in water and in water + methanol mixtures

✍ Scribed by Gehan M. El-Subruiti; Cecil F. Wells; Ibrahim M. Sidahmed

• Publisher: John Wiley and Sons
• Year: 1990
• Tongue: English
• Weight: 624 KB
• Volume: 22
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550220902

✦ Synopsis

Abstract

Rates of solvolysis of ions [Co(3Rpy)~4~Cl~2~]^+^ with R = Me and Et have been measured over a range of temperatures for a series of water‐rich water + methanol mixtures to investigate the effect of changes in solvent structure on the solvolysis of complexes presenting a largely hydrophobic surface to the solvent. The variation of the enthalpies and entropies of activation with solvent composition has been determined. A free energy cycle relating the free energy of activation in water to that in water + methanol is applied using free energies of transfer of individual ionic species from water into water + methanol. Data for the free energy of transfer of chloride ions Δ__G__(Cl^−^) from both the spectrophotometric solvent sorting method and the TATB method for separating Δ__G__(salt) into Δ__G__(i) for individual ions are used: irrespective of the source of Δ__G__(Cl^−^), in general, −Δ__G__(Co(Rpy)~4~Cl^2+^) > −Δ__G__(Co(Rpy)~4~Cl~2~^+^), where Rpy = py, 4Mepy, 4Etpy, 3Etpy, and 3Mepy, showing that changes in solvent structure in water‐rich water + methanol mixtures generally stabilize the cation in the transition state more than the cation in the initial state for this type of complex ion. A similar result is found when the free energy cycle is applied to the solvolysis of the dichloro (2,2′,2″‐triaminotriethylamine)cobalt(III) ion. The introduction of a Me or Et group on the pyridine ring in [Co(Rpy)~4~Cl~2~]^+^ has little influence on the difference {Δ__G__(Co(Rpy)~4~Cl^2+^)−Δ__G__(Co(Rpy)~4~Cl~2~^+^)} in water + methanol with the mol fraction of methanol < 0.20.