Explanation:
In chemistry, a ligand is a molecule or ion that forms a coordination complex by binding to a central metal ion. Ligands are typically Lewis bases, meaning they can donate a pair of electrons to form a coordinate bond. However, not all molecules or ions can act as ligands.

Overview of the options:
a) NO: This molecule can act as a ligand because it has a lone pair of electrons on the nitrogen atom, which can be donated to form a coordinate bond with a metal ion.
b) NH4: This molecule, also known as ammonium ion, is not expected to be a ligand. It does not have any lone pairs of electrons available for donation.
c) NH2CH2CH2NH2: This molecule, also known as ethylenediamine, can act as a ligand. It has two nitrogen atoms, each with a lone pair of electrons available for donation.
d) CO: This molecule, also known as carbon monoxide, can act as a ligand. It has a lone pair of electrons on the carbon atom, which can be donated to form a coordinate bond with a metal ion.

Explanation of the correct answer:
The correct answer is option 'b' - NH4. NH4 is not expected to be a ligand because it does not have any lone pairs of electrons available for donation. The nitrogen atom in NH4 is bonded to four hydrogen atoms, and all its valence electrons are involved in bonding. Therefore, NH4 cannot form coordinate bonds with metal ions.

In summary:
- NO, NH2CH2CH2NH2, and CO can act as ligands because they have lone pairs of electrons available for donation.
- NH4 is not expected to be a ligand because it does not have any lone pairs of electrons available for donation.