Interactions between metal-complex ions and water. Part 3.—Entropies of solution and heat capacity changes of metal-complex ions in water at 25 °C
Abstract
The entropies of solution and the heat-capacity changes of [ML6]3+, ΔS°s([ML6]3+) and ΔCp,s([ML6]3+), have been determined at 25 °C and are plotted against the polarization surface change densities, σp of [ML6]3+, where M is CoIII or CrIII, L is 1,3-propanediamine/2 (= tn/2), 1,2-ethanediamine/2 (= en/2), biguanide/2 (= bigH/2), urea, NH3 or H2O. Further, the values of ΔS°s[Cr(acac)3] and ΔCp,s[Cr(acac)3] in the literature have been included in the plots, where acac is the acetylacetonato ion. The values of –ΔS°s([ML6]n+) and ΔCp,s([ML6]n+)(n= 0 or 3), which are positive and large at σp= 0 of [Cr(acac)3], decrease as σp increases up to that of [Cr(urea)6]3+, followed by an increase. This suggests that [ML6]n+ changes the water structure in the vicinity of itself as its σp increases from zero, from a well ordered structure caused by hydrophobic hydration to a disordered one and them to a well ordered one caused by ionic (electrostrictive) hydration. [Cr(urea)6]3+ has an anomalously large ΔCp,s([ML6]n+) in the plot of ΔCp,s([ML6]n+) against σp, although no anomaly is observed in the plot of ΔS°s([ML6]n+)vs. σp. This anomaly is explained in terms of the transition between the hydrophobic and electrostrictive structures of water in the vicinity of [ML6]n+, which is similar to the order–disorder transition (λ-transition).