Typically, the following types of ligands are distinguished.
- Sigma-donor ligand. One that yields electrons to the metal center from its sigma orbitals.
- Pi-donor ligand. One that donates electron density to the metal center of the complex through its pi orbitals.
- Pi-acceptor ligand. One that has empty pi orbitals and receives electrons from the metal.
If these definitions are incomplete, or at worst incorrect, let me know.
Based on these definitions, you are asked to classify the following ligands into each of the three groups: H2O, en, Cl, PPh3, NH3, CO, η5-C5H5, Me3P, Ph, H.
I think it would be like this, but I don't quite know how to reason it out:
sigma-donor: H2O, NH3, Me3P
pi-acceptor: PPh3, Ph; n5-C5H5
The rest I wouldn't know, because although I know the definition, I don't know what to base it on to make the classification and justify it correctly.