Why is ammonia a good ligand?

Ammonia is a good ligand because its nitrogen atom has a lone pair of electrons that it can donate to a metal ion to make a stable coordinate bond.

Basic idea of ligands

In coordination chemistry, a ligand is any ion or molecule that donates an electron pair to a central metal ion. This electron sharing forms coordinate (dative) covalent bonds and creates a complex ion or coordination compound.

Ammonia's electron pair

The chemical formula of ammonia is NH₃. Nitrogen has five valence electrons. Three are used to bond with three hydrogen atoms. The remaining two form a lone pair. This lone pair sits in a high-energy, easily accessible orbital, ready to bond with a metal cation such as Cu²⁺, Ag⁺, Co³⁺, or Ni²⁺.

Why the bond is strong

• Size and shape: Ammonia is small, so several NH₃ molecules can crowd around one metal ion, often making octahedral complexes like [Cu(NH₃)₄]²⁺.
• Moderate basic strength: The lone pair is basic, meaning it is eager to share with an acidic metal ion, but it is not so strong that it breaks the metal-ligand balance.
• ?-donor ability: Ammonia donates electron density via a sigma bond, stabilizing high oxidation states of metals.
• No ?-backbonding: Because ammonia has no empty ?* orbitals, it does not compete for electrons that the metal might need elsewhere.

Consequences in everyday chemistry

The deep blue color of Schweizer's reagent, [Cu(NH₃)₄(OH)₂], shows ammonia's strong ligand nature. In qualitative analysis labs, the formation of [Ag(NH₃)₂]⁺ dissolves silver chloride, proving silver ions are present. In biochemistry, similar nitrogen-based ligands attach metal ions in enzymes.