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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?

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    $\begingroup$ If this is to work at all, I would suspect we would need a 4d or 5d transition metal in a low oxidation state. $\endgroup$ – Oscar Lanzi Mar 25 '18 at 16:56
  • $\begingroup$ @OscarLanzi why 4d or 5d? Shouldn't larger atomic radii be more stable due to increased distance of the positive ligands from the nucleus? $\endgroup$ – tox123 Mar 25 '18 at 17:00
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    $\begingroup$ I think a better pi donor metal would help, and 4d/5d with a low oxidation state is where I would look for that. $\endgroup$ – Oscar Lanzi Mar 25 '18 at 17:15
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    $\begingroup$ 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. $\endgroup$ – Mithoron Mar 25 '18 at 17:41
  • $\begingroup$ Complexes you mentioned are much to strongly protonated, related chemistry.stackexchange.com/questions/74528/… $\endgroup$ – Mithoron Mar 25 '18 at 17:54
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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.

enter image description here

The closest metal complex I could find where hydronium ion is present as an entity is hydronium zinc(II) pyrazine-2,3-dicarboxylate(again not the type of complex OP is looking) where 2,3-PZDC (μ-pyrazine-2,3-dicarboxylato-N,O,O′) act as ligand(emphasis mine):

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.

References

  1. Hydronium Ion Complex of 18-Crown-6:  Theory Confirms Three “Normal” Linear Hydrogen Bonds Michael Bühl, Ralf Ludwig, Rachel Schurhammer, and, and Georges Wipff, The Journal of Physical Chemistry A 2004 108 (51), 11463-11468 DOI: 10.1021/jp045879+
  2. Michal Gryz, Wojciech Starosta & Janusz Leciejewicz (2005) "Doubly bridged molecular ribbons in the structure of an ionic complex, hydronium zinc(II) pyrazine-2,3-dicarboxylate", Journal of Coordination Chemistry, 58:11, 931-935, DOI: 10.1080/00958970500055492
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