Ligands are molecules, ions, or species that form coordinate bonds with a central metal atom or ion in a coordination complex. These bonds involve the donation of electron pairs from the ligand to the metal center. Ligands play a crucial role in determining the structure, properties, and reactivity of coordination compounds. The interaction between the ligands and the central metal gives rise to the unique characteristics of these complexes.

Here are some key points about ligands:

1. Electron Pair Donors: Ligands contain atoms with lone pairs of electrons, which they share with the metal atom to form a coordinate covalent bond. The ligands are electron pair donors in this interaction.
2. Types of Ligands: Ligands can be classified based on their binding sites and the number of atoms involved in bonding:
   • Monodentate Ligands: Ligands that form one bond with the central metal atom (e.g., H2O, Cl-, NH3).
   • Bidentate Ligands: Ligands that can form two bonds simultaneously with the central metal (e.g., ethylenediamine, en).
   • Polydentate Ligands: Ligands that can form multiple bonds with the central metal, creating a chelate (e.g., ethylenediaminetetraacetate, EDTA).
3. Chelating Ligands: Some ligands, known as chelating ligands, are capable of forming multiple bonds to the central metal, creating a more stable ring-like structure called a chelate.
4. Naming Ligands: Ligands are named before the central metal in the name of a coordination compound. The names of ligands are often modified with prefixes like “di-” or “tri-” to indicate the number of ligands, and they may end in “-o” or “-a” in the case of anions.
5. Coordination Number: The coordination number of the central metal in a complex is determined by the number of ligands it can bond to. For example, in an octahedral complex, the coordination number is 6, meaning there are six ligands bonded to the central metal.
6. Examples of Ligands: Common ligands include water (H2O), ammonia (NH3), chloride ions (Cl-), cyanide ions (CN-), ethylenediamine (en), and many more.
7. Effect on Properties: The choice of ligands can greatly influence the color, stability, reactivity, and electronic properties of a coordination complex.
8. Ligand Field Theory: This theory explains how ligands influence the energy levels of the metal’s d orbitals, leading to the observed colors and properties of coordination complexes.

Ligands are a fundamental aspect of coordination chemistry, and their interactions with central metal atoms/ion form the basis for the wide range of structures and properties seen in coordination compounds.