Water-exchange reactions on the five-coordinate complex [Zn(H₂O)₄(L)]²⁺·2H₂O (L = water, methanol, ethanol, propan-1-ol, butan-1-ol, dimethyl ether, propan-2-ol, fluorophosgene, phosgene, formaldehyde, acetyl chloride, acetaldehyde, acetone and acetamide) were studied by quantum-chemical calculations (B3LYP/6-311+G**). The reactions follow an associative pathway that involves the formation of a six-coordinate intermediate [Zn(H₂O)₅(L)]²⁺·H₂O, followed by the dissociation of a water molecule to form the product [Zn(H₂O)₄(L)]²⁺·2H₂O. The water-exchange process involves isoenergetic cis- and trans-oriented transition states to form the product state that is similar to the reactant state. Of the studied ligands L, acetamide, which has the highest basicity, exhibited the highest activation energy and energy gap between the reactant and intermediate states. Electronic and steric effects of the coordinated ligands influence the activation barrier and the efficiency of the water-exchange process.

On the basis of DFT calculations (B3LYP/6-311+G**), the possibility to include solvent effects is considered in the investigation of the H₂O-exchange mechanism on [Be(H₂O)₄]²⁺ within the widely used cluster approach. The smallest system in the gas phase, [Be(H₂O)₄(H₂O)]²⁺, shows the highest activation barrier of +15.6 kcal/mol, whereas the explicit addition of five H-bonded H₂O molecules in [{Be(H₂O)₄(H₂O)}(H₂O)₅]²⁺ reduces the barrier to +13.5 kcal/mol. Single-point calculations applying CPCM (B3LYP(CPCM:H₂O)/6-311+G**//B3LYP/6-311+G**) on [Be(H₂O)₄(H₂O)]²⁺ lower the barrier to +9.6 kcal/mol. Optimization of the precursor and transition state of [Be(H₂O)₄(H₂O)]²⁺ within an implicit model (B3LYP(CPCM:H₂O)/6-311+G** or B3LYP(PCM:H₂O)/6-311+G**) reduces the activation energy further to +8.3 kcal/mol but does not lead to any local minimum for the precursor and is, therefore, unfavorable.