# Can Hydronium Act as a Ligand?

Can hydronium function as a ligand? For instance could there exist $\ce{[Al(H3O)4]^{+7}}$ or $\ce{[Fe(H3O)6]^{+9}}$ cations? I asked my chemistry teacher and he said that he's never encountered a coordination compound with cationic ligands (however I know that there are because $\ce{NO+}$ can act as a ligand). What are the properties of these cations, if they exist? How are they synthesized? And finally, how would you name these complexes?

• If this is to work at all, I would suspect we would need a 4d or 5d transition metal in a low oxidation state. – Oscar Lanzi Mar 25 '18 at 16:56
• @OscarLanzi why 4d or 5d? Shouldn't larger atomic radii be more stable due to increased distance of the positive ligands from the nucleus? – tox123 Mar 25 '18 at 17:00
• I think a better pi donor metal would help, and 4d/5d with a low oxidation state is where I would look for that. – Oscar Lanzi Mar 25 '18 at 17:15
• This is kinda like follow-up of chemistry.stackexchange.com/questions/93809/… Hydronium can be protonated by fluoroantimonic superacid, but only transiently, but even accepting h-bond makes it a ligand. – Mithoron Mar 25 '18 at 17:41
• Complexes you mentioned are much to strongly protonated, related chemistry.stackexchange.com/questions/74528/… – Mithoron Mar 25 '18 at 17:54