22 9 How does transition metal interact with ethene or ethylene (C2H4)?

Two d electrons from transition metal (TM) and the two p electrons from the C atoms may form two p-d sigma bonds. When determining the oxidation number of the metal, we assign the four sigma bonding electrons to the ligand (C2H4). Therefore, the TM has an oxidation number of +2.

Up to two d electrons of the metal may also flow into the ANTI-BONDING pi* orbital of ethene. This is called pi backbonding. The pi backbonding is not taken into account when we determine the oxidation numbers. Therefore, the metal has a zero oxidation number in this case.