Name: coordination complexes and chelation
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 51 s
Channel: JoVE

K12 Standards

new-york-state-science-learning-standards

New York State Science Learning Standards

High School

Physical Science

HS. Structure and Properties of Matter

HS-PS2-6.

Metal-Ligand Bonds

In a complex ion, the metal ion is bound to anionic or neutral molecules, known as ligands. Ligands have one or more lone pairs of electrons and act as electron-pair donors or Lewis bases. They donate the electron pair to the metal ion, which acts as an electron-pair acceptor or a Lewis acid. Thus, a coordinate covalent bond between the metal ion and ligand forms a Lewis acid-base adduct.
The ligand atom donating an electron pair is known as the donor atom. The number of donor atoms surrounding the metal ion correlates to the coordination number of the metal ion.
Depending upon the number of donor atoms present, ligands are further classified as monodentate, bidentate, or polydentate ligands, which can be charged or neutral.
Monodentate ligands have only one donor atom. Oxygen is a donor atom in a neutral water molecule; nitrogen is a donor atom in a charged cyanide ion.
Bidentate ligands have two donor atoms that are sufficiently far apart to bind simultaneously to a metal ion. The nitrogen atoms of three bipyridine molecules bind to a ruthenium ion with the coordination number six.
A polydentate ligand has two or more donor atoms present in its structure. Here, six nitrogens from two molecules of diethylenetriamine bind to a cobalt ion.
As multiple donor atoms appear to hold the central metal ion like a claw, the bidentate and polydentate ligands are also called chelating agents.
A chelating agent has more affinity for the central metal ion than a monodentate ligand, forming a more stable coordination complex. This is known as the chelating effect.
Chelating agents are versatile. They are used to complex interfering metal ions in a reaction&#8212;to increase the shelf life of food products by complexing trace metal ions involved in catalyzing decomposition reactions or treating lead poisoning using ethylenediaminetetraacetate or EDTA.
The complex ion formation influences the physicochemical properties of the metal ion and ligands, such as change in oxidation and reduction potentials, or colors. Furthermore, ligands can undergo reactions such as deprotonation of hydrated metal ions or displacement by another strongly-attracted ligand. Here, the water molecule is replaced by ammonia, which binds stronger to the metal ion.

metal-ligand bonds

The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain coordination compounds. Ions of the metals, especially the transition metals, are likely to form complexes.
In these complexes, transition metals form coordinate covalent bonds, a kind of Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base) to an electron acceptor (Lewis acid). The Lewis acid in coordination complexes, often called a central metal ion (or atom), is the transition metal or inner transition metal. The Lewis base donors, called ligands, can be a wide variety of chemicals&#8212;atoms, molecules, or ions. The only requirement is that they have one or more electron pairs, which can be donated to the central metal. Most often, this involves a donor atom with a lone pair of electrons that can form a coordinate bond to the metal. The coordination sphere consists of the central metal ion or atom plus its attached ligands. Brackets in a formula enclose the coordination sphere; species outside the brackets are not part of the coordination sphere.
Ligands are monodentate, from the Greek for &#8220;one-toothed,&#8221; when they connect with the central metal through only one atom. Here, the number of ligands and the coordination number are equal. Ligands with one donor atom, such as NH3, Cl&#8722;, and H2O, are monodentate ligands. Many other ligands coordinate to the metal in more complex fashions.
Bidentate ligands are those in which two atoms coordinate to the metal center. For example, ethylenediamine (en, H2NCH2CH2NH2) contains two nitrogen atoms, each of which has a lone pair and can serve as a Lewis base (Figure 1a). Both of the atoms can coordinate to a single metal center. In the complex [Co(en)3]3+, there are three bidentate en ligands, and the coordination number of the cobalt(III) ion is six (Figure 1b). The most common coordination numbers are two, four, and six, but examples of all coordination numbers from 1 to 15 are known.
<img alt="Chemical structures; (a) Ethylenediamine molecule; (b) Cobalt complex with ethylenediamine ligands." src="/files/ftp_upload/11456/11456_Image_1.svg" style="text-align: center; height: 200px;" data-altupdatedat="1761830439000" data-alt="Image1" />
Figure 1. (a) ethylenediamine (en) contains two nitrogen atoms, with a lone pair each, that can coordinate with a metal ion. (b) Three bidentate en ligands coordinate with a single cobalt ion.
Any ligand that binds to a central metal ion by more than one donor atom is a polydentate ligand (or &#8220;many teeth&#8221;) because it can bite into the metal center with more than one bond. The term chelate from the Greek for &#8220;claw&#8221; is also used to describe this type of interaction. Many polydentate ligands are chelating ligands, and a complex consisting of one or more of these ligands and a central metal is a chelate. A chelating ligand is also known as a chelating agent. A chelating ligand holds the metal ion rather like a crab&#8217;s claw would hold a marble. Polydentate ligands are sometimes identified with prefixes that indicate the number of donor atoms in the ligand. The heme complex in hemoglobin is another important example (Figure 2). It contains a polydentate ligand with four donor atoms that coordinate to iron.
<img alt="Heme structure formula, iron chelation diagram, molecule analysis, biochemistry study." src="/files/ftp_upload/11456/11456_Image2.svg" style="text-align: center; height: 200px;" data-altupdatedat="1761830439000" data-alt="Image2" />
Figure 2: The single ligand heme contains four nitrogen atoms that coordinate to iron in hemoglobin to form a chelate.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/19-2-coordination-chemistry-of-transition-metals">Openstax, Chemistry 2e, Chapter 19.2 Coordination Chemistry of Transition Metals</a>.

The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain ...

Education

JoVE Core

Chemistry

Transition Metals and Coordination Complexes

transition-metals-and-coordination-complexes

KEY TERMS AND CONCEPTS

coordination complexes and chelation

Coordination complexes form when a metal ion bonds to ligands. These compounds appear in familiar examples such as hemoglobin in blood, chlorophyll in green plants, vitamin B-12, and the catalyst used to make polyethylene. Transition metal ions are especially likely to form these complexes.

In a coordination complex, the metal acts as a Lewis acid. The ligand acts as a Lewis base and donates both electrons in the coordinate covalent bond. A ligand can be an atom, molecule, or ion, as long as it has one or more electron pairs to share with the metal. The central metal ion or atom, along with its attached ligands, makes up the coordination sphere. Brackets in a formula show the coordination sphere, and anything outside the brackets is not part of it.

Ligands can bind in different ways. Monodentate ligands connect through only one atom, so the number of ligands equals the coordination number. Common examples are NH3, Cl−, and H2O. Bidentate ligands bind through two atoms. Ethylenediamine, or en, has two nitrogen atoms with lone pairs that can both bond to one metal center. In the complex [Co(en)3]3+, three bidentate ligands give cobalt(III) a coordination number of six.

Ligands that attach through more than one donor atom are called polydentate ligands. The word chelate comes from a Greek word meaning “claw,” and it describes how these ligands hold a metal ion. A complex with one or more chelating ligands is called a chelate, and a chelating ligand is also called a chelating agent. These ligands can be identified with prefixes that show the number of donor atoms they use.

The heme complex in hemoglobin is another key example. Heme is a single polydentate ligand with four donor atoms, and all four coordinate to iron. Coordination numbers in complexes are often 2, 4, or 6, but examples from 1 to 15 are known. These bonding patterns help explain the structure of many important metal-containing compounds.

Coordination Complexes and Chelation

properties of transition metals

Transition Metals in the Periodic Table

Transition metals are the d-block elements and the inner transition metals in the f-block. They are defined by partially filled d orbitals. In the periodic table, the transition elements are in groups 3 through 11, and the inner transition metals appear in the two rows below the main body of the table. The d-block is often grouped into transition series. The first transition series runs from Sc through Cu. The second series runs from Y through Ag. The third series includes La and Hf through...

coordination compounds and nomenclature

How Transition Metal Complexes Are Named

Transition metal complexes are named by following rules for ligands, metal ions, and oxidation state. These compounds form when a central metal ion or atom, often a transition metal or inner transition metal, accepts electron pairs from ligands. A ligand is a Lewis base that can be an atom, molecule, or ion with one or more lone pairs to donate. In these complexes, the bond is a coordinate covalent bond. That means both electrons in the bond come from the donor, such as ammonia, and are...

Coordination complexes form when a metal ion bonds to ligands. These compounds appear in familiar examples such as hemoglobin in blood, chlorophyll in green plants, vitamin B-12, and the catalyst used to make polyethylene. Transition metal ions are especially likely to form these complexes. In a coordination complex, the metal acts as a Lewis acid. The ligand acts as a Lewis base and donates both electrons in the coordinate covalent bond. A ligand can be an atom, molecule, or ion, as long as...

coordination number and geometry

Transition Metal Shapes from Coordination Number

Transition metal complexes are often identified by their coordination number and the shape around the metal ion. The coordination number is the number of donor atoms bonded to the central metal. In many complexes, the most common shapes are octahedral, tetrahedral, and square planar. Coordination number and molecular geometry are linked in a clear pattern. A coordination number of 2 gives a linear shape, as in [Ag(NH3)2]+. A coordination number of 3 gives a trigonal planar shape, as in...

structural isomerism

Linkage and Ionization Isomers

Isomers are different chemical species that share the same chemical formula. In coordination compounds, structural isomerism includes linkage isomers and coordination-sphere isomers. These two forms show how the same atoms can be arranged in different ways around a metal center. Linkage isomers form when a ligand can attach to a transition metal through two different atoms. The cyanide ligand, CN−, can bind through carbon or nitrogen. The thiocyanate ligand, SCN−, can bind through sulfur or...

stereoisomerism

Isomer Shapes in Coordination Complexes

Isomer shapes in coordination complexes can change a compound’s properties even when the chemical formula stays the same. Isomers are different chemical species with the same formula. In transition metal complexes, the atoms stay connected through the same types of bonds, but their arrangement in space can differ. One common form is geometric isomerism. In these complexes, ligands (the ions or molecules attached to the metal) can sit in cis or trans positions. The octahedral [Co(NH3)4Cl2]+ ion...

valence bond theory

Metal Ligand Bonds in Coordination Complexes

Coordination complexes form metal-ligand bonds through valence bond theory. In this model, Linus Pauling explained bonding with hybridization and overlap of atomic orbitals. The central metal atom or ion acts as a Lewis acid. It uses empty orbitals of suitable energy to accept electron pairs from ligands, which act as Lewis bases. These coordinate covalent bonds help explain the color, geometry, and magnetic behavior seen in coordination compounds. The type of hybridization helps determine the...

crystal field theory - octahedral complexes

crystal-field-theory-octahedral-complexes

Octahedral d-Orbital Splitting in Crystal Fields

Crystal field theory explains how ligands affect the d orbitals in transition metal complexes. This electrostatic model helps predict the colors, magnetic behavior, and some structures of coordination compounds. It focuses on the nonbonding electrons on the central metal ion, not on the metal-ligand bonds. In crystal field theory, the metal ion and ligands are treated as very small point charges. Because electrons are negative, electrons donated by the ligands repel the electrons in the...

crystal field theory - tetrahedral and square planar complexes

crystal-field-theory-tetrahedral-and-square-planar-complexes

Crystal Field Splitting in Non-Octahedral Shapes

Crystal field theory also explains d orbital splitting in tetrahedral and square planar complexes. These shapes do not follow the same pattern as an octahedral complex, so the d orbitals respond to ligand placement in different ways. In tetrahedral complexes, the four ligands sit between the axes. None of the d orbitals points directly at the ligands, but the d x2−y2 and d z2 orbitals lie along the Cartesian axes and overlap less with the ligands than the d xy, d xz, and d yz orbitals. The d...

colors and magnetism

Color and Spin in Coordination Complexes

Color in coordination complexes comes from how metal electrons absorb light. When atoms or molecules absorb light at the right frequency, electrons move to higher-energy orbitals. For many main-group atoms and molecules, the absorbed light is ultraviolet and cannot be seen. In coordination compounds, the energy gap between d orbitals often matches visible light, so the compound can absorb and emit colors that the human eye detects. Small changes in orbital energy can cause large shifts in the...

key terms and concepts

Explore coordination complexes, ligand types, coordination numbers, and chelation using examples like hemoglobin, chlorophyll, and heme for high school chemistry.

Coordination Complexes and Chelation | JoVE K12

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HS-PS1-9.

Coordination Complexes and Chelation

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