How can NH3 be a ligand but NH4+ is not a ligand??
NH3 has an unoccupied lone pair of electrons. Those electrons can be donated to another atom that needs them; e.g. H to become NH4^+, but in doing so, it loses those two electrons and there is nothing to donate. Said another way, NH3 as a lone pair of electrons but NH4^+ does not.
To understand why NH3 can act as a ligand while NH4+ cannot, let's first define what a ligand is. In chemistry, a ligand is an atom, ion, or molecule that donates a pair of electrons to a central metal atom or ion, forming a coordinate bond.
NH3, also known as ammonia, is a molecule that consists of one nitrogen atom bonded to three hydrogen atoms. It has a lone pair of electrons on the nitrogen atom that can be donated to a central metal atom, making it an ideal ligand. When NH3 acts as a ligand, it forms a coordinate bond with the metal cation by donating its lone pair of electrons to the empty d orbitals of the metal.
On the other hand, NH4+ is a polyatomic cation, commonly known as ammonium ion. It is formed when NH3 accepts a proton (H+) to become NH4+. The addition of a proton to NH3 results in the loss of the lone pair of electrons on the nitrogen atom. As a result, NH4+ does not have a pair of electrons available to donate, making it unable to act as a ligand.
In summary, NH3 can act as a ligand because it has a lone pair of electrons that can be donated to a central metal atom, while NH4+ cannot act as a ligand since it does not have a pair of electrons available for donation.
NH3 can act as a ligand because it possesses a lone pair of electrons on the nitrogen atom, which can form a coordinate bond with a central metal ion. This lone pair of electrons allows NH3 to donate an electron pair to the metal ion, forming a coordination complex.
On the other hand, NH4+ cannot act as a ligand because it does not possess a lone pair of electrons. NH4+ is a positively charged ion, and all four hydrogen atoms are bonded to the central nitrogen atom, leaving no lone pairs available for bonding with a metal ion. Therefore, NH4+ cannot form coordinate bonds and does not function as a ligand.