[gilleain]

Cyclopentadiene is a great molecule - it can form 'metallocene' compounds where two cyclopentadiene (Cp) rings 'sandwich' a metal ion between them:

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

Like Cp--M--Cp where the '--' are an unusual kind of 'bond' which is somewhat like five carbon-metal bonds, although I'm sure there is a more accurate orbital description of the interaction.

[theideaofcoffee]

The ‘--‘ is a ligand bond, a fundamental aspect of coordination and organometallic chemistry.