The species, which is not a ligand is:

\(NO_2\)

The correct answer is option 4. \(NO_2\).

A ligand is a molecule or ion that binds to a central metal atom to form a coordination complex. It donates a lone pair of electrons to the metal, forming a coordinate bond.

\(NO_3^-\): This is a nitrate ion and can readily act as a ligand due to its negative charge and lone pair of electrons on the nitrogen atom.

\(NO\): This is nitric oxide and also possesses a lone pair of electrons on the nitrogen atom. It can function as a ligand and readily forms complexes with transition metals.

\(NO^+\): This is nitrosyl cation and also has a lone pair of electrons available for bonding. It can act as a ligand and forms complexes with various metals.

\(NO_2\): This is nitrogen dioxide and lacks a lone pair of electrons on the nitrogen atom. It forms only covalent bonds with other molecules and cannot readily coordinate with metal centers. Therefore, it doesn't fit the definition of a ligand.

While NO₂ participates in chemical reactions and bonding, it doesn't form the specific type of bond with metals required for it to be considered a ligand. So, (4) NO₂ is the species that is not a ligand among the given options.