In chemistry, a ligand is either an atom, ion, or molecule (see also: functional group) that bonds to a central metal, generally involving formal donation of one or more of its electrons. The metal-ligand bonding ranges from covalent to more ionic. Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known involving Lewis acidic "ligands."[1]


Ligands are classified in many ways: their charge, size (bulk), the identity of the coordinating atom(s), and their denticity. The size of a ligand is indicated by its cone angle.

Inner- vs out-sphere ligands

In coordination chemistry, the ligands that are directly bonded to the metal (that is, share electrons), are sometimes called "inner sphere" ligands. "Outer-sphere" ligands are not directly attached to the metal, but are bonded, generally weakly, to the first coordination shell, affecting the inner sphere in subtle ways. The complex of the metal with the inner sphere ligands is then called a coordination complex, which can be neutral, cationic, or anionic). The complex, along with its counter ions (if required), is called a coordination compound.

Metal-EDTA complex, wherein the aminocarboxylate is a hexadentate chelating ligand.

Strong field and weak field ligands

Main article: Crystal field theory

In general, ligands are viewed as donating electrons to the central atom. Bonding is often described using the formalisms of molecular orbital theory. In general, electron pairs occupy the HOMO of the ligands.

Ligands and metal ions can be ordered in many ways, one ranking system focuses on ligand 'hardness' (see also hard soft acid base theory). Metal ions preferentially bind certain ligands. In general, 'hard' metal ions prefer weak field ligands, whereas 'soft' metal ions prefer strong field ligands. From a MO point of view, the HOMO of the ligand should have an energy that overlaps with the LUMO of the metal preferential. Metal ions bound to strong-field ligands follow the Aufbau principle, whereas complexes bound to weak-field ligands follow Hund's rule.


3 orbitals of low energy: dxy, dxz and dyz
2 of high energy: dz2 and dx2y2



2 orbitals of low energy: dz2 and dx2y2
3 orbitals of high energy: dxy, dxz and dyz


The arrangement of the d-orbitals on the central atom (as determined by the 'strength' of the ligand), has a strong effect on virtually all the properties of the resulting complexes. E.g. the energy differences in the d-orbitals has a strong effect in the optical absorption spectra of metal complexes. It turns out that valence electrons occupying orbitals with significant 3d-orbital character absorb in the 400-800 nm region of the spectrum (UV-visible range). The absorption of light (what we perceive as the color) by these electrons (that is, excitation of electrons from one orbital to another orbital under influence of light) can be correlated to the ground state of the metal complex, which reflects the bonding properties of the ligands. The relative change in (relative) energy of the d-orbitals as a function of the field-strength of the ligands is described in Tanabe-Sugano diagrams.


Polydentate and polyhapto ligand motifs and nomenclature

Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have lone pairs on more than one atom. Ligands that bind via more than one atom are often termed chelating. A ligand that binds through two sites is classified as bidentate, and three sites as tridentate. The bite angle refers to the angle between the two bonds of a bidentate chelate. Chelating ligands are commonly formed by linking donor groups via organic linkers. The classic bidentate ligand is ethylenediamine, which is derived by the linking of two ammonia groups with an ethylene (-CH2CH2-) linker. A classic example of a polydentate ligand is the hexadentate chelating agent EDTA, which is able to bond through six sites, completely surrounding some metals. The number of atoms with which a polydentate ligand bind to the metal centre is called its denticity, symbolized κn, where n indicates the number non-contiguous donor sites by which a ligand attaches to a metal. EDTA4−, when it is sexidentate, binds as a κ6-ligand, the amines and the carboxylate oxygen atoms are not contiguous. In practice, the n value of a ligand is not indicated explicitly but rather assumed. The binding affinity of a chelating system depends on the chelating angle or bite angle.

Related to but distinct to from denticity is hapticity, symbolized η or eta. Hapticity refers to the number of contiguous atoms in a ligand that are attached to a metal. Butadiene forms both η2 and η4 complexes depending on the number of carbon atoms are bonded to the metal. To simplify matters, ηn usually refers to unsaturated hydrocarbons and κn usually to describe polydentate amine and carboxylate ligands.


Single atom bonding motifs

Ambidentate ligand

Unlike polydentate ligands, ambidentate ligands can attach to the central atom in two places but not both. A good example of this is thiocyanate, SCN, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to linkage isomerism. Polyfunctional ligands, see especially proteins, can bond to a metal center through different ligand atoms to form various isomers.

Bridging ligand

Bridging ligand link two or more metal centers. Polyatomic ligands such as CO22- are especially prone to bridge. The bonding is complicated because polyatomic ligands are ambidentate and thus the capacity for many different linkage isomers. Atoms that bridge metals are sometimes indicated with prefix of "μ" (mu). Most inorganic solids, e.g. FeCl2, are polymers by virtue of the presence of multiple bridging ligands.

Metal ligand multiple bond

Metal ligand multiple bonds some ligands can bond to a metal center through the same atom but with a different number of lone pairs. The bond order of the metal ligand bond can be in part distinguished through the metal ligand bond angle (M-X-R). This bond angle is often referred to as being linear or bent with further discussion concerning the degree to which the angle is bent. For example, an imido ligand in the ionic form has three lone pairs. One lone pair is used as a sigma X donor, the other two lone pairs are available as L type pi donors. If both lone pairs are used in pi bonds then the M-N-R geometry is linear. However, if one or both these lone pairs is non-bonding then the M-N-R bond is bent and the extent of the bend speaks to how much pi bonding there may be. η1-Nitric oxide can coordinate to a metal center in linear or bent manner.

Specialized ligand types

Noninnocent ligand

Noninnocent ligands bond with metals in such a manner that the distribution of electron density between the metal center and ligand is unclear. Describing the bonding of noninnocent ligands often involves writing multiple resonance forms which have partial contributions to the overall state.

Trans spanning ligand

Trans spanning ligands are bidentate ligands that can span opposite sites of a complex with square-planar geometry. A wide variety of ligands that chelate in the cis fashion already exist, but very few can link opposite verices on a coordination polyhedron. Early attempts to generate trans-spanning bidentate ligands relied on polymethylene chains to link the donor functionalities, but such ligands lead to coordination polymers.

A diphosphane linked with pentamethylene was claimed to span across a square planare complex. This early attempt was followed by ligands with more rigid backbones. "TRANSPHOS" was the first trans-spanning diphosphane ligand that usually coordinates to palladium(II) and platinum(I1) in a trans manner. TRANSPHOS features benzo[c]phenanthrene substituted by diphenylphosphinomethyl (Ph2PCH2) groups at the 1 and 11 positions.[2][3] The polycyclic framework suffers sterically clashing hydrogen centers. XANTHOS is a more reliable trans-spanning ligand.[2] without the steric problems associated with TRANSPHOS. SPANPHOS is comparable to XANTHOS.

Subsequent to the reports on SPANPHOS and related ligands was a genuine trans-spanning ligand reported, one that would form neither bimetallic nor oligomeric complexes with certain transition metals, and strictly function as a trans-chelator This ligand, TRANSDIP, represented the first trans-spanning ligand to give exclusively chelating complexes, even when reacted with d8 metal ion halides.[2] TRANSDIP is based on a α-cyclodextrin.[4]

Common ligands

See nomenclature.




Examples of common ligands (by field strength)


Iodide iodo I monoanionic monodentate
Bromide bromo Br monoanionic monodentate
Sulfide thio or bridging thiolate S2− dianionic monodentate (M=S), or bidentate bridging (M-S-M')
Thiocyanate thiocyanato S-CN monoanionic monodentate ambidentate (see also isothiocyanate, below)
Chloride chloro Cl monoanionic monodentate also found bridging
Nitrate O-NO2 monoanionic monodentate
Azide N-N2 monoanionic monodentate
Fluoride fluoro F monoanionic monodentate
Hydroxide hydroxo O-H monoanionic monodentate often found as a bridging ligand
Oxalate [O-C(=O)-C(=O)-O]2− dianionic bidentate
Water aqua H-O-H neutral monodentate monodentate
Isothiocyanate isothiocyanato N=C=S monoanionic monodentate ambidentate (see also thiocyanate, above)
Acetonitrile CH3CN neutral monodentate
Pyridine C5H5N neutral monodentate
Ammonia ammine NH3 neutral monodentate
Ethylenediamine en neutral bidentate
2,2'-Bipyridine bipy neutral bidentate easily reduced to its (radical) anion or even to its dianion
1,10-Phenanthroline phen neutral bidentate
Nitrite nitro N-O2 monoanionic monodentate ambidentate (see also nitrito)
Nitrite nitrito O-N-O monoanionic monodentate ambidentate (see also nitro)
Triphenylphosphine PPh3 neutral monodentate
Cyanide cyano CN monoanionic monodentate can bridge between metals (both metals bound to C, or one to C and one to N)
Carbon monoxide carbonyl CO neutral monodentate can bridge between metals (both metals bound to C)

Note: The entries in the table are sorted by field strength, binding through the stated atom (i.e. as a terminal ligand), the 'strength' of the ligand changes when the ligand binds in an alternative binding mode (e.g. when it bridges between metals) or when the conformation of the ligand gets distorted (e.g. a linear ligand that is forced through steric interactions to bind in a non-linear fashion).

Other general encountered ligands (alphabetical)

In this table other common ligands are listed in alphabetical order.

Acetonitrile MeCN neutral monodentate
Acetylacetonate (Acac) CH3-C(O)-CH-C(O)-CH3 monoanionic bidentate In general bidentate, bound through both oxygens, but sometimes bound through the central carbon only,
see also analogous ketimine analogues
Alkenes R2C=CR2 neutral compounds with a C-C double bond
Benzene C6H6 neutral and other arenes
1,2-Bis(diphenylphosphino)ethane (dppe) Ph2PC2H4PPh2 neutral bidentate
1,1-Bis(diphenylphosphino)methane (dppm) C25H22P2 neutral Can bond to 2 metal atoms at once, forming dimers
Corroles tetradentate
Crown ethers neutral primarily for alkali and alkaline earth metal cations
2,2,2-crypt hexadentate primarily for alkali and alkaline earth metal cations
Cryptates neutral
Cyclopentadienyl [C5H5] monoanionic
Dimethylglyoximate (dmgH) monoanionic
Ethylenediaminetetraacetate (EDTA) tetra-anionic hexadentate actual ligand is the tetra-anion
Ethylenediaminetriacetate trianionic pentadentate actual ligand is the trianion
glycinate bidentate other α-amino acid anions are comparable (but chiral)
Heme dianionic tetradentate macrocyclic ligand
Scorpionate ligand tridentate
Sulfite monoanionic monodentate ambidentate
2,2',5',2-Terpyridine (terpy) neutral tridentate meridional bonding only
Thiocyanate monoanionic monodentate ambidentate, sometimes bridging
see also the N,N',N"-trimethylated analogue
Tricyclohexylphosphine (C6H11)3P or (PCy3) neutral monodentate
Triethylenetetramine (trien) neutral tetradentate
Trimethylphosphine PMe3 neutral monodentate
Tris(2-diphenylphosphineethyl)amine (np3) neutral tetradentate
Terpyridine C15H11N3 neutral tridentate

See also


  2. ^ N. J. DeStefano, D. K. Johnson, R. M. Lane, L. M. Venanzi "Transition-Metal Complexes with Bidentate Ligands Spanning trans-Positions. I. The synthesis of 2,11-bis(diphenylphosphinomethyl)benzo[c]-phenanthrene, a ligand promoting the formation of square planar complexes" Helvetica Chimica Acta 1976, volume 59, pp. 2674-2682. doi:10.1002/hlca.19760590806
  3. ^ Mochida, J. A. Mattern, J. C. Bailar Jr., J. (1975) “Stereochemistry of Complex Inorganic Compounds.XXXV. A Complex Containing a Ligand That Spans Trans Positions” J. Am. Chem. Soc., volume 97, 3021–3026. doi:10.1021/ja00844a017.
  4. ^ L. Poorters, D. Armspach, M. Dominique, L. Toupet, S. Choua, P. Turek (2007) “Synthesis and properties of TRANSDIP, a rigid chelator built upon a cyclodextrin cavity: is TRANSDIP an authentic trans - spanning ligand?” Chemistry, A European Journal, volume 13, pp. 9448-9461. doi:10.1002/chem.200700831


The content of this section is licensed under the GNU Free Documentation License (local copy). It uses material from the Wikipedia article "Ligand" modified June 23, 2008 with previous authors listed in its history.