Variable-Pressure Kinetic and Equilibrium Study of the Complexation of the Beryllium(II) Ion with 4-Isopropyltropolone in Acidic Aqueous Solution

Kinetics and equilibria for the formation and dissociation of the 1:1 complex of the beryllium(II) ion with 4-isopropyltropolone (Hipt) in acidic aqueous solution have been studied spectrophotometrically at various temperatures and pressures at an ionic strength of 0.100 M ((Na,H)ClO₄). Equilibrium measurements gave the following results: K = [Be(ipt)⁺] [H⁺] [Be²⁺]⁻¹ [Hipt]⁻¹ = 1.17 ± 0.06 (25.0 °C), ΔH° = 2.4 ± 0.3 kJ mol⁻¹, ΔS° = 9.2 ± 1.1 J K⁻¹ mol⁻¹, ΔV° = +5.3 ± 0.2 cm³ mol⁻¹ (25.0 °C). The reaction volumes for the proton dissociation of Hipt and for the complexation of Be²⁺ with ipt⁻ are -8.2 ± 0.5 and +13.5 ± 0.6 cm³ mol⁻¹, respectively, at 25.0 °C. The rate constants and activation parameters for formation and dissociation of the complex are as follows: k_f = 58.1 ± 0.5 M⁻¹ s⁻¹ (25.0 °C), k_r = 49.8 ± 0.5 M⁻¹ s⁻¹ (25.0 °C), ΔH_f* = 38.1 ± 0.4 kJ mol⁻¹, ΔH,* = 35.7 ± 0.4 kJ mol⁻¹, ΔS_f* = -83.5 ± 1.3 J K⁻¹ mol⁻¹, ΔS_r* = -92.7 ± 1.2 J K⁻¹ mol⁻¹, ΔV_f* = −7.1 ± 0.2 cm³ mol⁻¹ (25.0 °C), ΔK_r* = −12.4 ± 0.2 cm³ mol⁻¹ (25.0 °C). The negative values of entropy and volume of activation indicate that the reaction proceeds with an associative mode of activation.