Hydrogen bonds in the second coordination sphere of metal complexes in the gas phase – playing by the rules?†

Hydrogen bonds in the second coordination sphere of metal complexes play a crucial role in the fine-tuning of their chemical and physical properties, including catalytic activity and selectivity. Our gas-phase computational study on hydrogen bonds of 180 aqua and ammine complexes of transition metals indicates that hydrogen bond energy depends on the charge of the complex, as well as on the ratio between the metal oxidation state (OS) and metal coordination number (CN), and is independent of the geometry of the complex, metal type and nature of other ligands. We have determined a linear increase in interaction energy with the increase in charge, as well as a linear increase of interaction energy with the increase in the OS/CN value. Based on the data presented in this work, we can predict and tune energies of hydrogen bonds in the second coordination sphere of metal complexes. That is, ligands of the same type in complexes with the same charge and the same OS/CN ratio will form hydrogen bonds with very similar energies, independent of all other factors.