# 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

This is not the answer OP would expect because I couldn't find metal complexes of type $$\ce{[M(H3O)_x]^{y+}}$$ (the reason for non-existence(?) is mentioned in the comments) but I did find a complex of an organic compound where hydronium ion act as a ligand i.e a complex of 18-crown-6 and hydronium ion(emphasis mine):
The observation that cyclic crown ethers can bind cations with high affinity and selectivity has had a tremendous impact in chemistry. The complex between 18-crown-6 (18c6) and the hydronium ion, $$\ce{H3O+}$$, is a prototypical representative of this important host−guest paradigm and has been the subject of numerous experimental and theoretical studies. The computational evidence presented so far has favored a structure with three linear $$\ce{O−H···O}$$ hydrogen bridges as the lowest, stable minimum, whereas a bifurcated arrangement has turned out to be a transition state for $$\ce{H3O+}$$ rotation 4−5 kcal/mol higher in energy.
The structure of dihydronium [catena-bis(μ-pyrazine-2,3-dicarboxylato-N,O,O′)zinc(II)], $$\ce{(H3O)2[Zn(2,3PZDC)2]}$$, is composed of polyanionic ribbons of zinc(II) ions linked by double bridging 2,3-PZDC ligand molecules. Each ligand uses an N,O bonding moiety formed by one carboxylic group [$$\ce{Zn–O}$$ 2.071(2) Å; $$\ce{Zn–N}$$ 2.184(2) Å] and a monodentate oxygen atom of the other carboxylate group [$$\ce{Zn–O}$$ 2.092(2) Å]. Coordination around the zinc(II) ion is strongly distorted octahedral. Hydronium cations $$\ce{H3O+}$$ link the ribbons by hydrogen bonds.