Are there any examples of tricoordinate hydrogen?

There are cases where hydrogen forms two covalent bonds (of order 0.5 I assume) in certain molecules and ions such as $\ce{H3^+}$, $\ce{CH5^+}$, $\ce{B2H6}$, $\ce{HF2^-}$, and hydrogen-bridged cations (below is an example from [wikipedia](https://en.wikipedia.org/wiki/Hydrogen-bridged_cations) about hydrogen-bridged cations), but are there cases where a coordination number of 3 or higher is achieved, even theoretically? Aside from maybe the more salt-like compounds, e.g. $\ce{NaH}$.


[![enter image description here](https://i.stack.imgur.com/sd6ul.png)](https://i.stack.imgur.com/sd6ul.png)




Magnesium hydride, with the rutile structure, has each hydrogen atom bonded to three magnesium atoms, but this is generally considered predominantly ionic (see [here](https://chemistry.stackexchange.com/a/31825/17175) for a discussion of the structure and ionicity of alkali and alkaline earth hydrides).


A better candidate for covalently bound, three-coordinate hydrogen is the iridium complexes given by [Chodosh et al.[1]](https://doi.org/10.1021/ic00134a005). The formulae of these complexes are described as follows:


$\ce{(IrH2LL')3(\mu\_3H)(BF4)2}$


$\ce{L: PPh3,P}\text{(cyclohexyl)}\_3$


$\ce{L':}\text{ pyridine, acetonitrile}$


In these complexes the three iridium centers are each bonded to two outer hydride ligands and rwo neutral organic ligands, with a central hydride ligand joined to all three iridium centers holding the complex together.


The introductory section of the article (available in front of the paywall) mentions several other, previously reported examples.


Three is actually not the maximum. [Cousins et al. [2]](https://doi.org/10.1039/C3CC47393G) have obtained four-coordinate hydrogen incorporated into alkali metal phenolate oligomers, the hydrogen taking the place of a lithium or sodium atom and thus showing alkali-metal-like characteristics.


**Reference2**


1. Daniel F. Chodosh, Robert H. Crabtree, Hugh Felkin, Sheila Morehouse, and George E. Morris (1982). "Trinuclear iridium cluster containing a tricoordinate bridging hydrogen ligand: structural and chemical studies". *Inorg. Chem.* **21**, 4, 1307–1311.
Publication Date:April 1, 1982
<https://doi.org/10.1021/ic00134a005>
2. David M. Cousins, Matthew G. Davidson and Daniel García-Vivó (2013). "Unprecedented participation of a four-coordinate hydrogen atom in the cubane core of lithium and sodium phenolates". *Chem. Commun.* **49**, 11809-11811.
<https://doi.org/10.1039/C3CC47393G>.


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