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 ΔC_p,s([ML₆]³⁺), have been determined at 25 °C and are plotted against the polarization surface change densities, σ_p of [ML₆]³⁺, where M is Co^III or Cr^III, 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 ΔC_p,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 ΔC_p,s([ML₆]ⁿ⁺)(n= 0 or 3), which are positive and large at σ_p= 0 of [Cr(acac)₃], decrease as σ_p 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 σ_p 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 ΔC_p,s([ML₆]ⁿ⁺) in the plot of ΔC_p,s([ML₆]ⁿ⁺) against σ_p, although no anomaly is observed in the plot of ΔS^°_s([ML₆]ⁿ⁺)vs. σ_p. 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).