page_title: Ligand
section_title: Bulky ligands
hierarchical_section_title: Ligand / Ligand motifs / Bulky ligands
caption_attribution_description: English: structure of a typical NHC
context_page_description: In coordination chemistry, 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 bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligands".
  Metals and metalloids are bound to ligands in virtually all circumstances, although gaseous "naked" metal ions can be generated in a high vacuum. Ligands in a complex dictate the reactivity of the central atom, including ligand substitution rates, the reactivity of the ligands themselves, and redox. Ligand selection is a critical consideration in many practical areas, including bioinorganic and medicinal chemistry, homogeneous catalysis, and environmental chemistry.
  Ligands are classified in many ways, including: charge, size, the identity of the coordinating atom, and the number of electrons donated to the metal. The size of a ligand is indicated by its cone angle.
context_section_description: Bulky ligands are used to control the steric properties of a metal center. They are used for many reasons, both practical and academic. On the practical side, they influence the selectivity of metal catalysts, e.g., in hydroformylation. Of academic interest, bulky ligands stabilize unusual coordination sites, e.g., reactive coligands or low coordination numbers. Often bulky ligands are employed to simulate the steric protection afforded by proteins to metal-containing active sites. Of course excessive steric bulk can prevent the coordination of certain ligands.
caption: The N-heterocyclic carbene ligand called IMes is a bulky ligand by virtue of the pair of mesityl groups.
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