Water‐Exchange Reaction of the Hexaaqua Ions of Vanadium(II), Manganese(II), and Iron(II) Revisited: A Discussion of Models with the Solvent Treated as a Dielectric Continuum

The H₂O‐exchange reaction on V(OH₂), Mn(OH₂), and Fe(OH₂) has been reinvestigated with ab initio quantum‐chemical calculations that include electron correlation and hydration, whereby the second coordination sphere and the bulk solvent were treated as a dielectric continuum. In such models, activation entropies (ΔS^≠) and also activation free energies (ΔG^≠) are not available, since the second coordination sphere is not treated quantum chemically. Furthermore, no transition states for the dissociative interchange (I_d) mechanism can be obtained, most likely also because of this approximation, and this limitation applies to the hexaaqua ions, but not, for example, to the pentaamines. Therefore, in cases where this model predicts that the dissociative (D) mechanism is the most favorable one, the question remains open as to whether the D or the I_d mechanism operates. For the H₂O adducts of the reactants, M(OH₂)₆⋅OH, two isomers were considered: in the first (A), the H₂O molecule in the second coordination sphere forms a single H‐bond to one aqua ligand, and in the other (B), it is bound to two aqua ligands in a bridging mode. For each hexaaqua ion, associative (I_a or A) and dissociative mechanisms (D) were investigated. On the basis of reactant B, activation energies agreeing with experiment were obtained for the water exchange on the aqua ions of V^II, Mn^II, and Fe^II. For the water exchange on V(OH₂) and Fe(OH₂), experimental and computational data suggest an a and a d activation, respectively, whereas for Mn(OH₂), the activation energies, calculated for the a and d activations, are equal and, therefore, the mechanism can be attributed only via the comparison of the change of the sum of all Mn^II−O bond lengths during the activation process, ΔΣd(Mn^II−O), with the activation volume. The limitations in the attribution of substitution mechanisms on the basis of experimental activation volumes and activation energies computed with the present model are analyzed. The electronic structure of the heptacoordinated species, the transition state V(OH₂)₅⋅⋅⋅(OH₂)]^≠ and the intermediate Mn(OH₂), are discussed.