Nitrogen dioxide (NO₂) is not typically considered a ligand in the traditional sense.

Here's why:

* Ligands typically bind to a metal center: Ligands are molecules or ions that donate electrons to a central metal atom or ion to form a coordination complex.

* NO₂ is a radical: Nitrogen dioxide is a free radical, meaning it has an unpaired electron. This makes it very reactive and unstable.

* NO₂ can act as an oxidizer: It is more likely to participate in oxidation reactions rather than binding to a metal center.

However, there are some exceptions:

* Nitrite ion (NO₂^-) is a common ligand: The nitrite ion, a reduced form of nitrogen dioxide, is a common ligand in coordination chemistry. It can bind to metal centers in a variety of ways, acting as a monodentate, bidentate, or bridging ligand.

In summary, while nitrogen dioxide itself is not a typical ligand, its reduced form, the nitrite ion, is a common ligand in coordination chemistry.