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Chelating Ligands: Definition, Models & Examples Video

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  • 0:04 What Are Chelating Ligands?
  • 2:19 Nomenclature - Rules &…
  • 3:52 Inner Metallic Complexes
  • 5:46 Lesson Summary
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Lesson Transcript
Instructor: Saranya Chatterjee

Saranya has a masters degree in Chemistry and in Secondary Education. She has taught high school, AP chemistry for 2 years and is teaching undergraduate college chemistry for 3 years.

This lesson will introduce chelating ligands, which are ligands with more than one donor site. It will provide the structures of some common ligands and go over nomenclature and the properties of chelate compounds.

What Are Chelating Ligands?

Ligands with two or more points of attachment to metal atoms are called chelating ligands, and the compounds they make are called chelates, a name derived from the Greek word khele, or claw of a crab. Non-chelating ligands such as ammonia (NH3) are monodentate, with one point of attachment (one tooth). Other ligands are described as bidentate because they have two points of attachment. A good example is ethylenediamine (NH2 CH2 CH2 NH2), which can bond to a metal ion through each of the two nitrogens. Chelating ligands can have several points of attachment.


Ethylenediamine
ethylenediamine


Chelating rings may have any number of atoms. The most common contain five or six atoms including the metal ion. Smaller rings have angles and distances that lead to strain, while larger rings frequently result in crowding. Some ligands can form more than one ring. Ethylenediaminetetraacetic acid (EDTA) can form five rings by using the four carboxylate groups and the two amine nitrogens.


Metal-EDTA complex
metal EDTA complex


Here's a brief look at some common chelating ligands, including their names, abbreviations, structures, binding sites, and chelate rings:

Name Abbreviation Structure Binding sites Chelate ring
Ethylenediamine (NH2 CH2 CH2 NH2) en
ethylenediamine
two nitrogen atoms
Complex with 5-membered chelate ring
null
Acetylacetone (CH3 COCH2 COCH3) acac
acetylacetone
two oxygen atoms
Complex with 6-membered chelate rings
null
Ethylenediaminetetraacetic acid (C10 H16 N2 O8) EDTA
EDTA structure
four oxygen and two nitrogen atoms
metal-EDTA complex with 6-membered rings
null
Dimethylglyoximate (C4 H8 N2 O2) DMG
dimethylglyoxime
two oxygen atoms
NiDMG complex with 5-membered rings
null
Oxalate (C2 O4 2-) ox
oxalate ion
two oxygen atoms
Oxalate
null
1,10 phenanthroline (C12 H8 N2) phen
phenanthroline
two nitrogen atoms
Complex with 5-membered rings
Bis(1,10-phenanthroline)silver(I)
2,2' bipyridine (C10 H8 N2) bpy
bipyridine
two nitrogen atoms
Complex with 5-membered rings
Tris(2,2`-bipyridine)dichlororuthenium(II)

Nomenclature - Rules and Examples

Chelating complexes can be, well, complex, so special prefixes are sometimes necessary to avoid confusion. The prefixes bis-, tris-, tetrakis-, etc. are substituted for di-, tri-, tetra-, etc. The names of the ligands are then put in parentheses following the prefix.

A good example is dimethylamine. If we didn't use bis- and tris-, dimethylamine could be either ''two methyls connected to an amine ((CH3)2 NH)'' or ''two methylamines ((CH3 NH2)2)''. That obviously won't do, so we use dimethylamine to mean '' two methyls, one amine'' and bis(methylamine) to mean ''two methylamines.''

While useful in any potentially ambiguous complex, bis- and tris- are always used when di-, tri-, etc. are already present in the name of the ligand. Please recall that ligands are named in alphabetical order and the metal is assigned its oxidation state in parentheses.

Consider, for example, [Co((NH2 CH2 CH2 NH2)2 Cl2]+ -- dichlorobis(ethylenediamine)cobalt(III). In this complex, there are two ligands present: the anionic ligand chlorine and the neutral chelating ligand ethylenediamine. The name ''ethylenediamine'' itself contains the term ''di-,'' so ''two ethylenediamine ligands'' is written as ''bis(ethylenediamine)''.

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