Is there an upper limit on the coordination number of a coordination compound? [duplicate]








**This question already has answers here**:
 
 




[Can an atom bond with more than 8 other atoms?](/questions/31950/can-an-atom-bond-with-more-than-8-other-atoms)

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From the [Gold Book](https://goldbook.iupac.org/html/C/C01331.html), coordination number is:



> 
> In an inorganic coordination entity, the number of σ-bonds between ligands and the central atom. π-bonds are not considered in determining the coordination number.
> 
> 
> 


In high school, we mostly had complexes upto coordination number six (like $\ce{[FeF6]^3-}$). But I wondered if the coordination number could go even higher.


From my search, I was able to reach [this research paper](https://pubs.acs.org/doi/abs/10.1021/ja00496a010?journalCode=jacsat) (*J. Am. Chem. Soc.*, **1979**, *101* (2), pp 334–340 DOI: 10.1021/ja00496a010) where they mention lanthanide complexes of coordination number uptil **nine**!


So, is **nine** the upper limit? Or can the coordination number go higher? Also, does the upper limit exist only *theoretically*, or has the corresponding complex been synthesized?




I don't see why not. It is a matter of having


* enough of an interaction between the center and the ligands, usually in the form of donor/acceptor orbital pairs, and
* enough space around the coordination center.


These go hand-in-hand, as any atom that will have enough electrons to donate to ligands will also most likely have a larger atomic radius than a first-row transition metal like iron.


For example, here is a thorium complex that has CN = 15 in its crystal structure and CN = 16 in an isolated gas-phase density functional calculation:


[![structure of [Th(H3BNMe2BH3)4]](https://i.stack.imgur.com/nwVQz.jpg)](https://i.stack.imgur.com/nwVQz.jpg)


[Daly, Scott R.; Piccoli, Paula M. B.; Schultz, Arthur J.; Todorova, Tanya K.; Gagliardi, Laura; Girolami, Gregory S. Synthesis and Properties of a Fifteen-Coordinate Complex: The Thorium Aminodiboronate $\ce{[Th(H3BNMe2BH3)4]}$. *Angew. Chem. Int. Ed.* **2010**, *49*, 3379-3381, DOI: 10.1002/anie.200905797](https://dx.doi.org/10.1002/anie.200905797)


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