Comment by gilleain

I meant metallocenes in general:

https://en.wikipedia.org/wiki/Metallocene

A metal atom sandwiched between two Cp rings. You _can_ model this as 5 single bonds between each atom of a ring (so 10 total C-M bonds), or you have to have some kind of 'edge' (bond) between the ring as a whole and the metal.

The more general issue is that a graph model of a chemical assumes a 'bond' is between exactly two atoms. Three-center hydrogen bonds are another example where this model fails to capture the chemistry very well.

Of course, it's a tradeoff - you can model _most_ compounds with just graphs (plus atom type, charge, chirality) and the relatively few that do not quite fit are special cases.