Mechanistic investigation of halopentaaquachromium(III) complexes: Comparison of computational and experimental results.

The mechanism for the substitution reactions of halopentaaquachromium(III) complexes in the series, [CrX(H 2 O) 5 ] 2+ , where X = F − , Cl − , Br − , or I − have been investigated using density functional theory. Several different mechanistic pathways were explored including associative interchange ( I a ), dissociative ( D ) and the associatively activated dissociation mechanisms ( D a ). The lowest overall activation enthalpy (ΔH ‡ ) obtained for the fluoride system is for the I a pathway, with immediate proton abstraction leading to the formation of HF and the conjugate base. For the chloride, bromide and iodide systems the D a pathway has the lowest ΔH ‡ values. Activation enthalpies determined at the PBE0/cc-pVDZ level, in aqueous solution (PCM), are in excellent agreement with the experimental results (MAD is 1.0 kJ mol −1 ). Reaction profiles were analyzed in terms of activation volumes to explain the observed trends. [ABSTRACT FROM AUTHOR]