Water Exchange Reactions and Hydrolysis of Hydrated Titanium(III) Ions. A Density Functional Theory Study

The structure and water exchange mechanism of hexahydrated Ti(III), its hydrolysis, and the water exchange mechanism of analogous hydroxo−aqua complexes have been studied using density functional theory (DFT) calculations. Isolated metal−aqua and metal−hydroxo clusters corresponding to the gas-phase ( T = 0 K) were used to approximate the model reactions. The structure of [Ti(H 2 O) 6 ] 3+ was found to have C i symmetry and Ti−O bond lengths of 2.094 Å. The water exchange reaction of this complex follows an (almost) limiting A mechanism with an energy of activation of 15.8 kcal mol -1 . The hydrolysis of hexahydrated Ti(III) was modeled by an in vacuo proton-transfer process between water molecules of the first and second coordination spheres of [Ti(H 2 O) 6 ] 3+ ·H 2 O. This process was found to be activationless and leads to the unusually stable dication:cation pair [Ti(H 2 O) 5 (OH)] 2+ ·H 3 O +, which is lower in energy than the reactant by 4.5 kcal mol -1 . Only a weak structural influence, indicated by a slight increase in the mean value of the Ti−O bond lengths of water molecules in the first coordination sphere, is observed when the hydroxo ligand is formed. The water exchange reactions of the corresponding hydroxo−aqua complexes [Ti(H 2 O) 5 (OH)] 2+ and [Ti(H 2 O) 5 (OH)] 2+ ·H 2 O, respectively, were found to proceed via limiting D mechanisms. The energies of activation for the exchange of the water molecule in the trans-position to the hydroxo ligand were calculated to be only 9.8 and 7.2 kcal mol -1, respectively. This, however, implies that the apparently weak influence of coordinated hydroxide still results in a significant reduction in the energy barrier for the water exchange reaction and also leads to a complete changeover in the preferred exchange pathway from A to D.