Co-ordination number and ligands

Co-ordination Number and Ligands

In coordination compounds, a central metal atom or ion is surrounded by ions or molecules called ligands. These ligands are bonded to the metal atom through coordinate bonds (donating lone pairs of electrons).

Co-ordination Number

The co-ordination number refers to the total number of donor atoms from ligands that are directly attached to the central metal ion. It plays a key role in deciding the shape of the coordination complex.

Examples:

• The complex [Cr(NH₃)₆]³⁺ has a coordination number of 6, as six ammonia molecules are directly bonded to the chromium ion.
• In [Ag(NH₃)₂]⁺, the coordination number is 2.

Common Coordination Numbers and Their Geometries

• 2: Linear (e.g., [Ag(NH₃)₂]⁺)
• 4: May show either tetrahedral or square planar geometry (e.g., [Ni(CN)₄]²⁻)
• 6: Octahedral (e.g., [Fe(CN)₆]³⁻)

Ligands

Ligands are ions or molecules that can donate at least one lone pair of electrons to the central metal ion to form a coordinate bond.

Types of Ligands

• Monodentate: Bind through one donor atom (e.g., Cl⁻, NH₃, H₂O)
• Bidentate: Bind through two donor atoms (e.g., ethylenediamine)
• Polydentate: Ligands that can attach to the central metal ion through more than two donor atoms (e.g., EDTA)
• Ambidentate: Ligands that can bind through two different atoms, but only one at a time (e.g., NO₂⁻)

Importance in JEE Main

Understanding coordination number and ligands is important for predicting complex structures, IUPAC naming, and electronic configurations—topics commonly tested in JEE Main.