Name: isomer shapes in coordination complexes
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 52 s
Channel: JoVE

Stereoisomers of coordination compounds are molecules that have the same chemical formula and the same connectivity of the ligands to the metal atom but differ in the arrangement of the ligand molecules around the central atom.
Based on the molecular structure, stereoisomers of coordination compounds are classified as geometric isomers or optical isomers.
Geometric isomers are stereoisomers that have completely different molecular geometries and exist with distinct physical and chemical properties.
cis&#8211;trans isomers are an example of geometric isomers. They occur in square-planar complexes with two sets of two identical ligands, or MA2B2, and in octahedral complexes with a set of four identical ligands and a pair of another ligand, or MA4B2. 
For example, in the cis isomer of the square-planar complex diamminedichloroplatinum(II), known as cisplatin, the chloride ligands are adjacent to each other and on the same side of the molecule. In the trans isomer, transplatin, the chloride ligands are on the opposite sides of the molecule. 
Octahedral complexes with two sets of three identical ligands, or MA3B3, can exhibit another type of geometric isomerism, known as fac&#8211;mer isomerism. 
For example, in the fac isomer of triamminetrichlorocobalt(III), the three amine ligands and three chloride ligands are on opposite sides of the molecule and thus make up opposite faces of the octahedron. In the mer isomer, three of the same ligands form an arc around the central metal atom.
The other class of stereoisomers is defined as optical isomers. Like our hands, optical isomers are mirror images of each other and non-superposable. These isomers, also called enantiomers, are described as chiral and have very few differences in their physical and chemical properties.
Optical isomers are distinguished by their interaction with plane-polarized light. Polarized light has electric field vectors oscillating in a single plane, which is rotated by a certain amount when the polarized light passes through a solution of an enantiomer. 
For example, one enantiomer of tris(ethylenediamine)cobalt(III) rotates the plane to the right and is called dextrorotatory, or the d optical isomer. The other enantiomer rotates the plane to the left and is called levorotatory, or the l optical isomer.

stereoisomerism

<h3>Isomerism in Complexes</h3>
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis configuration, the two chloride ligands are adjacent to each other. The other isomer, the trans configuration, has the two chloride ligands directly across from one another.
<img alt="cis-trans isomerism diagram of Co(III), Cl/H2O ligands; violet cis form, green trans form." src="/files/ftp_upload/11459/11459_Image1.svg" style="text-align: center; height: 200px;" data-altupdatedat="1761830439000" data-alt="Image1" />
Figure 1. The cis and trans isomers of [Co(H2O)4Cl2]+ contain the same ligands attached to the same metal ion, but the spatial arrangement causes these two compounds to have very different properties.
Different geometric isomers of a substance are different chemical compounds. They exhibit different properties, even though they have the same formula. For example, the two isomers of [Co(NH3)4Cl2]NO3 differ in color; the cis form is violet, and the trans form is green. Furthermore, these isomers have different dipole moments, solubilities, and reactivities. As an example of how the arrangement in space can influence the molecular properties, consider the polarity of the two [Co(NH3)4Cl2]NO3 isomers. Remember that the polarity of a molecule or ion is determined by the bond dipoles (which are due to the difference in electronegativity of the bonding atoms) and their arrangement in space. In one isomer, cis chloride ligands cause more electron density on one side of the molecule than on the other, making it polar. For the trans isomer, each ligand is directly across from an identical ligand, so the bond dipoles cancel out, and the molecule is nonpolar.
Another important type of isomers are optical isomers, or enantiomers, in which two objects are exact mirror images of each other but cannot be lined up so that all parts match. This means that optical isomers are non-superposable mirror images. A classic example of this is a pair of hands, in which the right and left hand are mirror images of one another but cannot be superposed. Optical isomers are very important in organic and biochemistry because living systems often incorporate one specific optical isomer and not the other. Unlike geometric isomers, pairs of optical isomers have nearly identical properties (boiling point, polarity, solubility, etc.). Optical isomers differ only in the way they affect polarized light and how they react with other optical isomers. For coordination complexes, many coordination compounds such as [M(en)3]n+ [in which Mn+ is a central metal ion such as iron(III) or cobalt(II)] form enantiomers, as shown in Figure 2. These two isomers will react differently with other optical isomers. For example, DNA helices are optical isomers, and the form that occurs in nature (right-handed DNA) will bind to only one isomer of [M(en)3]n+ and not the other.
<img alt="Coordination compounds; chemical structure; diagram; metal-ligand complexes; molecular geometry." src="/files/ftp_upload/11459/11459_Image_2.svg" style="text-align: center; height: 200px;" data-altupdatedat="1761830439000" data-alt="Image2" />
Figure 2. The complex [M(en)3]n+ (Mn+ = a metal ion, en = ethylenediamine) has a non-superposable mirror image.
The [Co(en)2Cl2]+ ion exhibits geometric isomerism (cis/trans), and its cis isomer exists as a pair of optical isomers (Figure 3).
<img alt="cis-trans isomerism in coordination compounds, structural diagram, optical isomers, cobalt complex" src="/files/ftp_upload/11459/11459_Image_3.svg" style="text-align: center; height: 200px;" data-altupdatedat="1761830439000" data-alt="Image3" />
Figure 3. Three isomeric forms of [Co(en)2Cl2]+ exist. The trans isomer, formed when the chlorines are positioned at a 180&#176; angle, has very different properties from the cis isomers. The mirror images of the cis isomer form a pair of optical isomers, which have identical behavior except when reacting with other enantiomers.
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>

Stereoisomerism

Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric ...

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Transition Metals and Coordination Complexes

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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 has two geometric isomers. In the cis form, the two chloride ligands are next to each other. In the trans form, the chlorides are directly across from one another.

These geometric isomers can behave very differently. The two isomers of [Co(NH3)4Cl2]NO3 have different colors. The cis form is violet, and the trans form is green. They also differ in dipole moment, solubility, and reactivity. Polarity depends on bond dipoles and how they are arranged in space. In the cis isomer, the chloride ligands create more electron density on one side of the ion, so it is polar. In the trans isomer, matching ligands are opposite each other, so the dipoles cancel and the ion is nonpolar.

Another important category is optical isomerism, also called enantiomerism. Enantiomers are mirror images that cannot be superposed, like right and left hands. These optical isomers are important in organic chemistry and biochemistry because living systems often use one optical isomer but not the other. Unlike geometric isomers, enantiomers usually have nearly identical boiling points, polarity, and solubility. They differ in how they affect polarized light and how they react with other optical isomers.

Many coordination compounds also form enantiomers. A complex such as [M(en)3]n+, where M is a metal ion like iron(III) or cobalt(II) and en is ethylenediamine, can exist as a pair of non-superposable mirror images. These forms react differently with other optical isomers. For example, DNA helices are optical isomers, and natural right-handed DNA binds to only one isomer of [M(en)3]n+.

Some complexes show both geometric and optical isomerism. The [Co(en)2Cl2]+ ion has cis and trans forms. Its trans isomer has the chlorines 180° apart and has very different properties from the cis form. The cis isomer also exists as a pair of optical isomers. These mirror-image forms behave the same in most ways, except when they react with other enantiomers.

Isomer Shapes in Coordination Complexes

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...

metal-ligand bonds

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...

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...

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

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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

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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 cis, trans, and optical isomers in coordination complexes. See how ligand arrangement changes color, polarity, and reactivity in transition metal compounds.

Isomer Shapes in Coordination Complexes | JoVE K12

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Isomer Shapes in Coordination Complexes

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