A ligand is an ion or molecule that attaches to a central metal atom or ion in a metal complex by donating at least one pair of electrons.
Key Points about Ligands:
- Electron donor
- Ligands have lone pairs of electrons (on atoms like O, N, S, P, or C) that can be donated to the metal.
- The bond formed is a coordinate (dative) covalent bond.
- Types of ligands (by denticity = number of donor atoms):
- Monodentate – bind through one atom (e.g., H₂O, NH₃, Cl⁻, CN⁻).
- Bidentate – bind through two atoms (e.g., ethylenediamine, en; oxalate, C₂O₄²⁻).
- Polydentate (chelating ligands) – bind through multiple atoms (e.g., EDTA can bind through six donor atoms).
- Types (by charge):
- Neutral ligands – NH₃, H₂O, CO, PR₃.
- Anionic ligands – Cl⁻, OH⁻, CN⁻, NO₂⁻.
- Cationic ligands (less common) – e.g., NO⁺.
- Special ligands:
- π-acceptor ligands (like CO, phosphines) can accept electron density back from the metal.
- π-donor ligands (like halides, OH⁻) donate electrons strongly.
Examples of Complexes with Ligands:
- [Co(NH₃)₆]³⁺ → NH₃ acts as a neutral monodentate ligand.
- [Fe(CN)₆]³⁻ → CN⁻ is a negatively charged monodentate ligand.
- [Ni(en)₃]²⁺ → ethylenediamine (en) is a bidentate ligand.
In simple terms:
A ligand is like a “donor partner” that attaches to a metal center by giving its electrons, helping form a stable metal–ligand complex.