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.
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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