Ethylene is capable of acting as a ligand as the C=C π bond can donate electron density to an empty metal d orbital, forming a σ bond. A filled metal d orbital is capable of donating electron density into the C=C π antibonding orbital.
View Ethylene Molecular Orbitals here
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Carbon Monoxide | Phosphine | Hydrogen | Ethylene | Cyclobutadiene | Butadiene | Benzene | Allyl | Cyclopentadienyl | Carbene