Structures With Bidentate Ligands

Bidentate ligands are Lewis bases that donate two pairs ("bi") of electrons to a metal atom.

Bidentate ligands are often referred to as chelating ligands ("chelate" is derived from the Greek word for "claw") because they can "grab" a metal atom in two places.

A complex that contains a chelating ligand is called a chelate.

Some Bidentate Ligands
ethylenediammine
(en)		acetylacetonate ion
(acac)
phenanthroline
(phen)		oxalate ion
(ox)

= Ni

= C

= N

= H

= Cl

ethylenediammine (en)
Ethylenediammine is a neutral molecule containing two N atoms that can each donate a pair of electrons to a metal atom.		Ni(en)2Cl2
In this complex, two ethylenediammine molecules are bonded to the Ni atom. The coordination number of 6 results in an octahedral structure.

= Ni

= C

= O

oxalate ion (ox)
Oxalate ion is a bidentate ligand even though it contains four O atoms which have lone pairs of electrons.		[Ni(ox)2]2-
In this complex, two oxalate ions are bonded to the Ni atom. The coordination number of 4 results in a square planar structure.

= Rh

= C

= H

= N

= Cl

phenanthroline (phen)
Phenanthroline is a neutral molecule containing two N atoms that can each donate a pair of electrons to a metal atom.		[Rh(phen)2Cl2]+
In this complex, two phenanthroline molecules are bonded to the Rh atom. The coordination number of 6 results in an octahedral structure.

= Cr

= C

= H

= O

acetylacetonate ion (acac)
Acetylacetonate ion contains two O atoms which allow this ligand to function as a bidentate ligand.		Cr(acac)3
In this complex, three acetylacetonate ions are bonded to the Cr atom. The coordination number of 6 results in an octahedral structure.

Applications

ZUD cleanser, which contains oxalic acid, is used to remove rust deposits. Rust reacts with oxalic acid to produce a colorless, water-soluble complex ion (i.e., [Fe(C2O4)3]3-) which contains the bidentate ligand, oxalate ion. Because the complex ion is water-soluble it can be washed away.

= Fe

= O

= C

oxalate ion (ox)
Oxalate ion is a bidentate ligand even though it contains four O atoms which have lone pairs of electrons.		 [Fe(C2O4)3]3-
In this complex, three oxalate ions are bonded to the Fe atom. The coordination number of 6 results in an octahedral structure.