why NH4+ ion is not strong ligand
It does not have a free pair of electrons. This means it cannot form a dative covalent bond to a transition metal, forming a complex ion.
NH4+ is also positive and will therefore be repelled by the metal ion
The strength of a ligand is determined by its ability to form a coordinate bond with a metal ion by donating a pair of electrons. In the case of the NH4+ ion, it does have a lone pair of electrons on the nitrogen atom that can potentially form a coordinate bond with a metal ion. However, NH4+ is not a strong ligand compared to other ligands like chloride ions (Cl-) or ammonia (NH3).
To determine the strength of a ligand, we consider several factors:
1. Size of the donor atom: In the NH4+ ion, the donor atom is nitrogen, which is relatively small. Smaller donor atoms can form stronger bonds with metal ions because their lone pairs are closer to the metal ion, resulting in stronger electrostatic interactions.
2. Charge density on the donor atom: The charge density on the donor atom affects its ability to donate electrons. In the case of NH4+, the nitrogen atom has a formal charge of +1, which reduces its electron-donating capacity compared to a neutral molecule like NH3.
3. Molecular shape: The tetrahedral shape of the NH4+ ion places the lone pair of electrons on nitrogen further away from the metal ion. This increases the distance between the donor atom and the metal ion, weakening the bond.
4. Polarizability: Ligands that are more polarizable can form stronger bonds with metal ions. In the case of NH4+, the ammonium ion, the positive charge on the nitrogen atom reduces its polarizability, making it a weaker ligand.
Therefore, due to a combination of factors such as size, charge density, molecular shape, and polarizability, the NH4+ ion is considered a weak ligand compared to other ligands.