# Coordination compounds with alkene ligands

I came across this compound: $\ce{[Pd(η^2-C2H4)Cl2(H2O)]}$

What does that '$\eta^2$' sign mean? I am guessing that it means that the ligand is bidentate because if it wasn't, I don't think it is possible (well it would be rare) for a metal to be only co-ordinates by 3 ligands.

If this is the case, how can the carbon atom in ethene bond to Pd as it doesn't have a lone pair? Does the double break and hence the 2 carbons bond to the metal?

• I took the liberty of changing your title. See, it's not really about nomenclature, it's more about the nature of this compound. – Ivan Neretin Oct 1 '15 at 8:52
• ‘because if it wasn't, I don't think it is possible (well it would be rare) for a metal to be only co-ordinates by 3 ligands.’ There are two chlorines, water and ethylene. Assuming ethylene be bidentate, that would be five ligands; otherwise four. Perfectly normal for palladium. Also, complexes such as $\ce{[Ag(CN)2]-}$ (two ligands) exist. – Jan Oct 1 '15 at 17:32

It is not carbon that is donating a pair - instead, it is a $\pi$ bond as a whole. Its two electrons, which were shared between two carbons, are now shared between them and palladium in a kind of multicentered bond.
There is a huge class of compounds where organic molecules are attached to a metal in the same way. With larger conjugated $\pi$-systems, we may have ligands of $\eta^5$ coordination (look up ferrocene) and more, all the way to $\eta^8$ and maybe beyond that.