Answer

Which Metal d-Orbitals Are Involved in π-Bonding with Ligands?

In coordination chemistry, π-bonding between a metal and ligand arises when there is orbital overlap involving the metal’s d-orbitals and the ligand’s π or π* orbitals. This type of bonding is especially important in complexes involving ligands such as carbon monoxide (CO), ethene (C₂H₄), and cyanide (CN⁻).

Metal d-Orbitals Involved in π-Bonding

The metal d-orbitals most commonly involved in π-bonding are:

• d_xy
• d_xz
• d_yz

These orbitals are oriented between the Cartesian axes and can effectively overlap with the side-on π or π* orbitals of ligands.

Why These Orbitals?

These specific d-orbitals participate in π-bonding due to their directional compatibility with the π systems of ligands. For example:

• The d_xz and d_yz orbitals can overlap with π* orbitals lying in the xz or yz planes.
• The d_xy orbital overlaps with π* orbitals in the xy plane.

Role in Backbonding

In ligands such as CO, the metal donates electron density from its d_π orbitals into the empty π* (antibonding) orbitals of the ligand. This phenomenon is known as π-backbonding or metal-to-ligand π-donation.

Examples of Ligands That Accept π-Electron Density

• CO – Carbon monoxide (a classic π-acceptor)
• CN⁻ – Cyanide ion
• NO – Nitric oxide
• C₂H₄ – Ethene (in metal-olefin complexes)

Conclusion

In summary, the d_xy, d_xz, and d_yz orbitals of transition metals are the primary orbitals that engage in π-bonding with ligand orbitals. This bonding is crucial in many organometallic and coordination compounds where π-backbonding stabilizes the metal-ligand interaction.