Question

What type of ligand is ammonia?

Answer 1

Hint: Ligands are the molecule or we say ions which are attached to a central metal atom through a coordinate bond. The bonding between the ligand and central metal atom is coordinate bonding. Ligands transfer a pair of electrons to the central metal atom and thus ligands behave as a Lewis base and therefore metal atoms behave as Lewis acid.

Complete answer:
Since we know ligands are attached to the central metal atom by a coordinate bond. Ligands donate the electron pair to the central metal atom to establish a coordinate bond. Depending on the number of pairs of electrons it contributes to the central metal atom. We classify ligands as: $3.$
$1.$ Monodentate ligand: They donate a single pair of electrons to the central metal atom. Thus they form a single coordinate bond with metal atoms.
$2.$ Bidentate ligand: They donate two pairs of electrons to the central atom. Thus they form two coordinate bonds with metal atoms.
$3.$ Polydentate ligand: They donate more than two pairs of electrons to the central metal atom.
Thus we can see the structure of ammonia, there is only one lone pair which is present on it. Therefore it can donate only a single pair of electrons to the central metal atom.
Another classification is based on the capability of ligands to pair up the unpaired electron of the central metal atom. These are strong field ligands and weak field ligands. Ammonia is a strong field ligand because it can pair the all unpaired electrons of a central metal atom. This is also because of nitrogen.

Note:
The number of electrons a ligand donates to a central metal is the sum of all atoms present in the molecule which donate the electrons. These atoms are called the density of the molecule. If a molecule can donate three pairs of electrons from three atoms then its called denticity is three. Also weak field ligands have not much energy to pair up the unpaired electrons of the central metal atom.