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?

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    $\begingroup$ Theoretically speaking, coordination number is not a thing at all, especially when we look at lanthanides with their ugly coordination polyhedra. The maximum possible coordination number is not a physical constant of any significance. Practically speaking, I vaguely remember hearing of CN up to 14 in uranium borohydrides or something. $\endgroup$ Apr 16 '18 at 9:18
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    $\begingroup$ See Can an atom bond with more than 8 other atoms? $\endgroup$
    – ron
    Apr 16 '18 at 14:11
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    $\begingroup$ Note: I self-marked the question as duplicate after having two CVs and then going through the linked question. The thing is - even if the titles do not match - the excellent, well-detailed answers to that question contain exactly the stuff I am looking for, and it makes little sense to report them over here. $\endgroup$ Apr 16 '18 at 15:58

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]

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

  • $\begingroup$ Hmm, thanks! A small query though, "any atom that will have enough electrons to donate to ligands" isn't it in the other direction? Ligands donate electrons to metals, and not vice-versa? (pardon my basic knowledge, but that's what I was taught) $\endgroup$ Apr 16 '18 at 14:34
  • $\begingroup$ See metal-to-ligand charge transfer. $\endgroup$ Apr 16 '18 at 15:47

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