Ligand

Ligand

A ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from covalent to ionic. Furthermore, the metal-ligand bonding in coordination complexes can be characterized using the formalisms of molecular orbital theory.

Types of Ligands

Ligands are classified in many ways, including: charge, size, and the identity of the coordinating atom(s).

Anionic and Cationic Ligands

Ligands can be anions such as chloride ions, or cations such as hydronium ions. Anionic ligands are often referred to as ligand anions, and cationic ligands are often referred to as ligand cations.

Hard and Soft Ligands

Ligands can also be classified as hard or soft. Hard ligands tend to be small, nonpolarizable ions or molecules, which bind to hard metal ions. Soft ligands are generally larger, more polarizable ions or molecules that bind to soft metal ions.

Applications of Ligands

Ligands are used in many areas of chemistry and biology. In coordination chemistry, ligands are used to control the reactivity of the metal center, to which they are bound. In biochemistry, ligands are often small molecules that bind to proteins and other biomolecules, thereby altering their function and behavior.

See Also

• Coordination complex
• Metal-ligand multiple bond
• Chelation

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