Hydrogen bond is the most abundant noncovalent interaction of water molecules. Previous studies have shown that the strength of the water–water hydrogen bond (-5.0 kcal/mol) increases if one of the water molecules coordinates to transition metal (-9.7 kcal/mol). One of the indicators of this strengthening is the shortening of H∙∙∙O hydrogen bond distances in the crystal structures deposited in the Cambridge Structural Database (CSD).

To study how the coordination of both water molecules influences their hydrogen bonds, we used the ConQuest program to screen high-quality crystal structures deposited in the CSD. The contacts between two aqua ligands in these crystal structures were accepted as hydrogen bonds if the O∙∙∙O distance was shorter than 4.0 Å and the O-H∙∙∙O angle was bigger than 110°. The energies of hydrogen bonds were calculated using the B97D/def2-TZVP level of theory.

The majority of the obtained hydrogen bonds have H∙∙∙O distances between 1.8 Å and 2.0 Å, which is longer than hydrogen bonds between coordinated and free water (1.6 – 1.8 Å). Although this might point towards the weakening of hydrogen bonds by the coordination of both water molecules, the DFT calculations show that the energy of a single hydrogen bond between aqua ligands reaches -11.0 kcal/mol, most likely due to increased dispersion effects. We found that the observed increase in the lengths of hydrogen bonds between aqua ligands is induced by the size of aqua complexes and their tendency to form multiple simultaneous hydrogen bonds. Our DFT calculations show that the supramolecular structures with multiple hydrogen bonds between aqua ligands reach an interaction energy of ‑70.0 kcal/mol.

This study implies that the coordination of both water molecules further increases the strength of their hydrogen bonds and shows that hydrogen bonds between aqua ligands are significant contributors to the stability of supramolecular systems of metal complexes.