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In coordination chemistry, ligand is an ion or molecule (functional group) that binds to a central atom to form a coordination complex. In general, ligands are viewed as electron donors and the metals as electron acceptors, i.e., respectively, Lewis bases and Lewis acids, and the bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs. An atom in ligands that can provide lone pair electrons to form a coordination bond with the central atom is called a coordination atom. All the coordination atoms have lone pair electrons in the outermost electron layer, and the most common ones are non-metallic elements with greater electronegativity, such as nitrogen, oxygen, carbon, sulfur and halogens. Ligands have an incomparable position in the complex, they and their complexes have a wide range of applications.

LigandsFig.1 The ball-and-stick model of a ligand


Ligands play a key role in modifying and controlling metal complexes. Ligands can not only adjust the electronic properties of metal complexes, but also affect their space environment, thus, affecting and deciding the applications of metal complexes. On the one hand, different fields require a variety of complexes with ground and excited state properties, which can be achieved by selecting ligands with different π electron-acceptor and σ electron-donor abilities. On the other hand, the complexes are often required some specific functions in some application fields, and these functions are determined by the ligands. For examples, in the research of photoelectric chemistry, the complexes need to be firmly adsorbed on the surface of semiconductor electrode or colloidal particles, which requires the ligands with strong adsorption groups such as carboxyl, hydroxyl and phosphoric acid groups; Complexes used in the biological probes, the ligands need to have large planar aromatic structures, and when complexes used as luminescent ion sensors, such as pH sensors, the ligands should have pH-sensitive groups. In conclusion, ligands play an important role in coordination chemistry, the metal complexes with different properties and functions can be easily realized through the selection and design of ligands.


The ligands and their complexes have been widely used in industrial production, energy development, biochemistry, analytical chemistry and many other fields. Some representative applications of ligands are listed below.

  • Catalysis: Catalysis is ubiquitous in chemical industry of all kinds. Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture. Most catalysts in industrial production are metal complexes, which indispensable require the use of ligands. The metal complex catalysts synthesized by ligands can be used in a variety of catalytic fields such as homogeneous catalysis, asymmetric catalysis, photocatalysis, electrocatalysis, biocatalysis, and there are hundreds of reactions involved such as substitution reactions, addition reactions, elimination reactions, cycle rearrangement reaction, redox reactions, etc.
  • Ligands

  • Separations science: Ligands with chelating properties can form different properties chelates with various metal ions, so as to reduce and control the concentration of metal ions and separate them from solution, which is widely used in hydrometallurgy processes, extraction and separation of metal elements and water softening. In addition, the ordered structures obtained from transition metal complexes with ligands can be used in stereoselective syntheses while their selectivity toward actinides and lanthanides can be utilized in the removal of radio nuclides from aqueous solutions, thereinto, phosphorus-based ligands and macrocyclic ligands are two representative ligands for separations of radionuclides from aqueous solutions[1].
  • Biochemistry: In biological science, ligands and metal complexes both have  biological performance, and metal complexes have higher biological performance rates than ligands alone as metal systems can penetrate cells more easily. anyhow, the ligands made undeniable contributions. In this direction, numerous metal complexes have been investigated with medical effects and are in practice as drugs. Among them, cisplatin (cis-[PtCl2(NH3)2] acts as an anticancer chemotherapy drug which was developed in starting phase of metal complexes in biological applications. Beyond that, ligands and metal complexes can be used as antibacterial agents, antioxidant agent and antiviral agents[2].
  • Ligands


Ligands are classified in many ways, including: charge, size (bulk), the identity of the coordinating atom(s), and the number of electrons donated to the metal (denticity or hapticity). More frequently, according to coordination atom, ligands can be classified as phosphine ligand, oxygen-donor ligand, nitrogen-donor ligand, sulfur-donor ligand, carbon-donor ligand, olefin ligands, etc.

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  • Alexandratos S.D. and Natesan S. Coordination chemistry of phosphorylated calixarenes and their application to separations science[J]. Industrial & Engineering Chemistry Research, 2000, 39(11), 3998-4010.
  • Singh A.; et al. Exploring coordination preferences and biological applications of pyridyl-based organochalcogen (Se, Te) ligands[J]. Coordination Chemistry Reviews, 2022, 450, 214254.

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