STUDIES ON ACETATOPENTAAQUOCHROMIUM(3) ION

The kinetics of the acid-assisted, chromium(II)-catalyzed and vanadium(II)-catalyzed aquations of the acetatopentaaquochromium(III) ion were investigated, as well as the rate of isotope exchange between this ion and chromium(II). The analysis of the kinetic data for the acid-catalyzed aquation suggests that substantial protonation of the coordinated acetate takes place. At 25°, μ = 4.00, the protonation constant is 0.25 ± 0.10 F-1 (ΔH ≈ 1 kcal/mol). The specific rate constant (25°, μ = 4.00) for the aquation of the protonated acetatochromium(III) ion is (95 ± 5) × 10-6 sec-1 (ΔH≠ 18.6 ± 1.0 kcal/mol), and the specific rate constant for the unprotonated ion is (0.74 ± 0.02) × 10-6 sec-1 (ΔH≠ varies from 13 to 33 kcal/mol as the temperature varies from 25 to 55°). Chromium(II)-catalyzed aquation occurs predominantly by a path inverse in (H+). With the acid dissociation constant of the acetatopentaaquochromium(III) ion at 25° and μ = 1.00 measured as (2.9 ± 0.7) × 10-5 F (ΔH = 5.5 ± 1.5 kcal/mol), the specific rate constant corresponding to the activated complex of composition Cr2 + • Cr(OH)OAc+ is 0.85 ± 0.20 M-1 sec-1 (ΔH≠ = 16 ± 3 kcal/mol), and that for Cr2 + • CrOAc2+ is (1.56 ± 0.08) × 10-6 M-1 sec-1 (ΔH≠ = 26.7 ± 1.5 kcal/mol). The Cr2 +-Cr(OH)OAc2+ isotope-exchange reaction is inverse first order in acid, implying that the electron transfer proceeds via a double bridge; at 25°, μ = 1.00, kex = (5.8 ± 0.2) × 10-4 sec-1 (ΔH≠ = 18.9 ± 0.9 kcal/mol). The vanadium(II)-catalyzed aquation is a complex reaction, being inhibited by acetic acid, chromium(II), and vanadium(III). At high concentrations of vanadium(II) (≧0.07 F) the rate law is - d(CrOAc2+)/dt = (V2+)(CrOAc2+)(k0 + A′/(H+)) with k0 = 0.163 ± 0.009 M-1 sec-1 and A′ = (6.7 ± 1.4) × 10-3 sec -1 at 25°, μ = 1.00. © 1968, American Chemical Society. All rights reserved.