ligand
A molecule or ion bonded to another. It is most often used to describe species bonded to the central metal ion in a co-ordination compound.
Portions of the summary below have been contributed by Wikipedia.
Examples of common ligands (by field strength)
In the following table the ligands are sorted by field strength (weak field ligands first):
| Ligand | formula (bonding atom(s) in bold) | Charge | Most common denticity | Remark(s) |
|---|---|---|---|---|
| Iodide | I- | monoanionic | monodentate | |
| Bromide | Br- | monoanionic | monodentate | |
| Sulphide | S2- | dianionic | monodentate (M=S), or bidentate bridging (M-S-M') | |
| Thiocyanate | S-CN- | monoanionic | monodentate | ambidentate (see also isothiocyanate, vide infra |
| Chloride | Cl- | monoanionic | monodentate | also found bridging |
| Nitrate | O-NO2- | monoanionic | monodentate | |
| Azide | N-N2- | monoanionic | monodentate | |
| Fluoride | F- | monoanionic | monodentate | |
| Hydroxide | O-H- | monoanionic | monodentate | often found as a bridging ligand |
| Oxalate | [O-C(=O)-C(=O)-O]2- | dianionic | bidentate | |
| Water | H-O-H | neutral | monodentate | monodentate |
| Isothiocyanate | N=C=S- | monoanionic | monodentate | ambidentate (see also thiocyanate, vide supra) |
| Acetonitrile | CH3CN | neutral | monodentate | |
| Pyridine | C5H5N | neutral | monodentate | |
| Ammonia | 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 | O-N-O- | monoanionic | monodentate | ambidentate |
| Triphenylphosphine | PPh3 | neutral | monodentate | |
| Cyanide | CN- | monoanionic | monodentate | can bridge between metals (both metals bound to C, or one to C and one to N) |
| Carbon monoxide | 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.
| Ligand | formula (bonding atom(s) in bold) | Charge | Most common denticity | Remark(s) |
|---|---|---|---|---|
| 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 | |
| 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 | ||
| Diethylenetriamine (dien) | neutral | tridentate | related to TACN, but not constrained to facial complexation | |
| 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) | ||
| Haem | dianionic | tetradentate | macrocyclic ligand | |
| Nitrosyl | NO+ | cationic | bent (1e) and linear (3e) bonding mode | |
| Scorpionate ligand | tridentate | |||
| Sulfite | monoanionic | monodentate | ambidentate | |
| 2,2',5',2-Terpyridine (terpy) | neutral | tridentate | meridional bonding only | |
| Thiocyanate | monoanionic | monodentate | ambidentate, sometimes bridging | |
| Triazacyclononane (tacn) | (C2H4)3(NR)3 | neutral | tridentate |
macrocyclic ligand see also the N,N',N"-trimethylated analogue |
| Triethylenetetramine (trien) | neutral | tetradentate | ||
| Tris(2-aminoethyl)amine (tren) | neutral | tetradentate | ||
| Tris(2-diphenylphosphineethyl)amine (np3) | neutral | tetradentate | ||
| Terpyridine | neutral | tridentate |
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