Formation of Pi-bonds in Octahedral Complexes
To understand which ligand orbital forms pi-bonds in octahedral complexes, we need to consider the molecular orbital theory. In octahedral complexes, the metal center is surrounded by six ligands in an octahedral arrangement. These ligands can interact with the metal center through sigma bonds and pi-bonds.

Key Points:
- In octahedral complexes, the ligand orbitals that can form pi-bonds are the pi-acceptor orbitals.
- Pi-acceptor ligands are typically ligands with empty low-energy orbitals that can accept electrons from the metal center.
- Ligands such as carbon monoxide (CO) and cyanide (CN-) are known to be strong pi-acceptor ligands.
- Pi-bonding occurs when the pi-acceptor ligand donates its electron density into the empty d-orbitals of the metal center.

Explanation:
In an octahedral complex, the metal center has five d-orbitals available for bonding. When a pi-acceptor ligand approaches the metal center, it can interact with the metal's d-orbitals in a sideways overlapping fashion, forming pi-bonds. This interaction involves the donation of electron density from the pi-acceptor ligand into the empty d-orbitals of the metal center.
The formation of pi-bonds in octahedral complexes enhances the stability and reactivity of the complex. Pi-bonding can influence various properties of the complex, such as its color, magnetic behavior, and reactivity towards other molecules.
In summary, pi-bonds in octahedral complexes are formed by the interaction between pi-acceptor ligands and the d-orbitals of the metal center. This interaction plays a crucial role in the overall structure and properties of the complex.