Studies on cobalt-tris(bipyridyl) complexes in acetonitrile–water mixtures
Abstract
The
Gibbs energies of transfer, ΔtG°, for [Co(bipy)3]3+ and [Co(bipy)3]2+ in the full range of acetonitrile–water
mixtures were derived from solubility measurements of the picrates and partitioned into single-ion
contributions based on the tetraphenylarsonium tetraphenylborate assumption. The enthalpies of solution
were obtained from calorimetry of the perchlorates of [Co(bipy)3]3+ and [Co(bipy)3]2+. The single-ion transfer
entropies were calculated ia the tetraphenylarsonium tetraphenylborate assumption. In addition
polarography, cyclic voltammetry and UV–visible spectroscopy were also performed on these complexes in water–acetonitrile mixtures. Gibbs energies of transfer of [Co(bipy)3]+ were derived from the ΔtG° values
for [Co(bipy)3]2+ and from electrochemical measurements. The TΔtS° values for [Co(bipy)3]3+ and[Co(bipy)3]2+
were calculated from the enthalpies and the Gibbs energies of transfer. The TΔtS° values of [Co(bipy)3]+
were obtained from the respective data for [Co(bipy)3]2+ and from the assumption of a negligible thermal diffusion potential. The Gibbs energies of transfer are accounted for in terms of hydrogen–hydrogen
interactions between the methyl group of acetonitrile and the hydrogen atoms of the ligands of the studied
complexes. The interpretations of TΔtS° values and the enthalpies of transfer were based on different
regions for the structure of acetonitrile–water mixtures and on the possibility of
the Co–bipy cations
entering cavities
in the water structure of water-rich mixtures.