STOICHIOMETRY AND KINETICS OF AQUATION OF (H2O)5CRCHX22+ IONS (X = CL, BR, I) IN ACIDIC SOLUTIONS

Mechanistic studies of the aquation of pentaaquo(dihalomethyl)chromium(III) ions (halo: chloro, bromo, iodo) have been made at hydrogen ion concentrations of 0.02-1.0 M in the absence of oxygen and light. For (H2O)5CrCHCl22+ and (H2O)5CrCHBr22+ the observed rate follows the equation -d In [complex]/dt = k[complex], where k = 4 X 10-6 s-1 (ΔH‡ = 24.4 kcal/mol, ΔS‡ = 1.3 ± 1.1 eu) for CrCHCl22+ at 298 K and k = 1.4 X 10-6 s-1 (ΔH‡ = 27.3 kcal/mol, ΔS‡ = 6.3 ± 4.6 eu) for CrCHBr22+ at 298 K. The iodo complex showed a mild inverse hydrogen ion dependence, and the rate law has the form (k0 + k1[H+]-1)[CrCHI22+] where k0 = 8.5 X 10-8 (ΔH‡ = 2.73 kcal/mol, ΔS‡ = 0.8 ± 1.7 eu) and k1 = 2.0 × 10-9 s-1 (ΔH‡ = 28.9 kcal/mol, ΔS‡ = -3.2 ± 3.4 eu) at 298 K. The products of aquation are Cr(OH2)63+, halide ion, CO, HCO2H, and H2. For deuterated complexes, CrCDX22+, HD is the major hydrogen product and no kinetic isotope effect is detected. A mechanism is proposed in which a chromium-formyl intermediate, (H2O)5CrCOH2+, is formed by eliminating two halide ions by an SN2 attack of H2O at the carbon. © 1979, American Chemical Society. All rights reserved.