Is there an upper limit on the coordination number of a coordination compound? From the Gold Book, 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 (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:



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