Stoichiometry and kinetics of aquation of (H2O)5CrCHX22+ ions (X = chlorine, bromine, iodine) in acidic solutions

Abstract

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 (H20)5CrCHCl22+ and (H20)5CrCHBr22+ the observed rate follows the equation -d In [complex]/di = ^[complex], where k = 4 X 10-6 s'1 (AH* = 24.4 kcal/mol, AS* = 1.3 ± 1.1 eu) for CrCHCl22+ at 298 K and k = 1.4 X 10-6 s-1 (AH* = 27.3 kcal/mol, AS* = 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 + /t1[H+]"*)[CrCHI22+] where k0 = 8.5 X 10-8 s-1 (AH* = 2.73 kcal/mol, AS* = 0.8 ± 1.7 eu) and k¡ = 2.0 X 10~9 s'1 (AH* = 28.9 kcal/mol, 5* = -3.2 ± 3.4 eu) at 298 K. The products of aquation are Cr(OH2)63+, halide ion, CO, HC02H, and H2. For deuterated complexes, CrCDX22+, HD is the major hydrogen product and no kinetic isotope effect is detected (kH/kD = 1). A mechanism is proposed in which a chromium-formyl intermediate, (H20)5CrC0H2+, is formed by eliminating two halide ions by an SN2 attack of H20 at the carbon.