Types of Ligands

A ligand is an ion or a molecule capable of functioning as an electron donor and therefore participates in forming a coordinate bond. The ligands are of different types.

Unidentate ligand

A unidentate ligand is one which uses only one atom at a time as the donor atom; this fill only one coordination position of a given cation. The ligands such as Cl^–, Br^-, and I^-, which contain only one atom each, are necessarily unidentate or monodentate. Polyatomic ligands such as CN^–, SCN^–, NO₃^– and NO₂^– are monodentate. Molecular ligands such as R3N, pyridine, CO and H₂O are also monodentate.

Polydentate ligand

A polydentate ligand uses more than one atom per ligand as donor atoms; such a ligand simultaneously fills more than one coordination position of a given cation. For example, the oxalate ligand ^–OOC-COO^– is a bidentate ligand, as it can form two coordinate bonds with a cation.

oxalate-ions

The carbonate ion (carbonato) is a bidentate ligand, ^–O-CO-O^–. Diethylenetriamine is a tridentate ligand.

\[\displaystyle {{H}_{2}}N-C{{H}_{2}}-C{{H}_{2}}-NH-C{{H}_{2}}-N{{H}_{2}}\]

Triethylenetetramine is a quadridentate ligand.

\[\displaystyle {{H}_{2}}NC{{H}_{2}}C{{H}_{2}}NHC{{H}_{2}}C{{H}_{2}}NHC{{H}_{2}}C{{H}_{2}}N{{H}_{2}}\]

Formula (Neutral molecules)	Name of the Ligand	Formula (Anions)	Name of the Ligand
H₂O	Aqua	F^-	Fluoro
NH₃	Ammine	Cl^-	Chloro
CO	Carbonyl	Br^-	Bromo
NO	Nitrosyl	I^-	lodo
CH₂NH₂	Methylamine	HO^-	Hydroxo
C₅H₅N	Pyridine	CN^-	Cyano
PPh₃	Triphenylphosphine	O₂^-	Oxo
NH₂NH₂	Hydrazine	SO₄^2-	Sulphato
Et₂O	Diethylether	S₂O₃^2-	Thiosulphato
		NO₂^-	Nitro
		ONO^-	Nitrito
		SCN^-	Thiocyanato
		NCS^-	Isothiocyanato
		NO₃^-	Nitrato
		N₃^-	Azido
		CH₃COO^-	Acetato
		H^-	Hydrido

The anion of ethylenediaminetetraacetic acid (EDTA) is a hexadentate ligand.

Ethylenediaminetetraacetic acid- EDTA

When a polydentate ligand binds a cation, the resulting complex has a ring structure; such a complex is called a chelate.

Bridging ligand

A bridging ligand is a ligand which simultaneously binds two metals; it forms a bridge between the two metals, acting as a ligand to both metals.

bridging-ligand-Pd₂Cl₂

Formula	Name of the ligand	Ligancy
CO₃^2-	Carbonato	2
(COO^-)₂	Oxalato	2
H₂NCH₂COO^-	Glycinato	2
(C₅H₄N)₂	Dipyridyl	2
H₂NCH₂CH₂NH₂	Ethylenediamine	2
H₂N(CH₂)₂NH(CH₂)₂NH₂	Diethlenetriamine	3
NC₅H₄C₅H₃N.C₅H₄N	2,2", 2"-Terpyridine	3
N(CH₂CO^-)₃	Nitrilotriacetato	4
H₂N(CH₂)₂NH(CH₂)₂NH(CH₂)₂NH₂	Triethylenetetramine	4
(^-OOCCH₂)₂N(CH₂)₂NHCH₂COO^-	Ethylenediaminetriacetato	5
N(CH₂CH₂NH₂)₃	Triethlaminetriamine (tripod ligand)	4
(^-OOCCH₂)₂N(CH₂)₂N(CH₂COO^-)₂	Ethylenediaminetetraacetato	6

Ambidentate ligand

An ambidentate ligand is capable of bonding through one type of donor atom in one complex but through a different donor atom in another complex. Such a ligand has potentially two ligating atoms. For example, the ligand NO₂^– can bond to a metal either through its O or through its N atom, producing two isomeric complexes.

\[\displaystyle \left[ {{{{\left( {N{{H}_{3}}} \right)}}_{5}}Co-ONO} \right]C{{l}_{2}}RED\]

\[\displaystyle [{{(N{{H}_{3}})}_{5}}Co-N{{O}_{2}}]C{{l}_{2}}Yellow\]

Other ambidentate ligands are CN^– cyano (M-CN) and isocyano (M-NC); S₂O₃^2- thiosulphato-S(M-SSO₃) and thiosulphato-O(M-OSO₂S); and also M-SCN thiocyanato and M-NCS isothiocyanato.

Flexidentate ligand

A flexidentate ligand is a polydentate ligand which can bind to metals with different numbers of its ligand sites. For example, though EDTA usually functions as a hexadentate ligand, it can sometimes function as a pentadentate or a tetradentate ligand. Therefore, it is called a flexidentate ligand. Similarly, SO can function either as a monodentate or as a bidentate ligand.