Isomerism, Equilibria and Biological Systems

Cis and trans configurations are possible in some octahedral and square planar complexes. In addition to these geometrical isomers, optical isomers (molecules or ions that are mirror images but not superimposable) are possible in certain octahedral complexes. Coordination complexes have a wide variety of uses including oxygen transport in blood, water purification, and pharmaceutical use.

34.1 Isomerism in Complexes

Learning Objectives

By the end of this section, you will be able to:

Explain and provide examples of geometric and optical isomerism
Identify several natural and technological occurrences of coordination compounds

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. In the cis configuration, the two chloride ligands are adjacent to each other (Figure 34.1). The other isomer, the trans configuration, has the two chloride ligands directly across from one another.

Figure 34.1

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.

Two structures are shown. The first is labeled, “Violet, cis form.” Below this label inside brackets is a central C o atom. From the C o atom, line segments indicate bonds to a C l atom above and the O atom of an H subscript 2 O group below the structure. Above and to both the right and left, dashed wedges with their vertex at the C o atom widening as they move out from the atom indicate bonds with O atoms of H subscript 2 O groups. Similarly, solid wedges below to both the right and left indicate bonds to a C l atom on the right and the O atom of an H subscript 2 O group on the left. This structure is enclosed in brackets. Outside the brackets to the right is the superscript plus sign. The second is labeled, “Green, trans form.” Below this label inside brackets is a central C o atom. From the C o atom, line segments indicate bonds to C l atoms above and below the structure. Above and to both the right and left, dashed wedges indicate bonds with O atoms of H subscript 2 O groups. Similarly, solid wedges below to both the right and left indicate bonds to the O atoms of H subscript 2 O groups. This structure is also enclosed in brackets with a superscript plus sign outside the brackets to the right.

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.

EXAMPLE 34.1.1

Geometric Isomers

Identify which geometric isomer of [Pt(NH₃)₂Cl₂] is shown in Figure 34.3. Draw the other geometric isomer and give its full name.

Solution

In the Figure 34.3, the two chlorine ligands occupy cis positions. The other form is shown in Figure 34.2. When naming specific isomers, the descriptor is listed in front of the name. Therefore, this complex is trans-diamminedichloroplatinum(II).

Figure 34.2

The trans isomer of [Pt(NH₃)₂Cl₂] has each ligand directly across from an adjacent ligand.

A structure is shown with a central P t atom. From this atom, a dashed wedge with its vertex at the P t extends up and to the left toward the C l atom, indicating a bond, and widening as it moves out. Another dashed wedge with its vertex at the P t extends up and to the right toward the N H 3 atom, indicating a bond, and widening as it moves out. Similarly, a solid wedge below and to the left indicates a bond with an H 3 N atom and another solid wedge below and to the right indicates a bond with a C l atom.

Check Your Learning

Draw the ion trans-diaqua-trans-dibromo-trans-dichlorocobalt(II).

Answer

The structure in this figure shows a structure inside brackets. A central C o atom is shown with line segments indicating bonds to C l atoms above and below the structure. Above and to the left, a dashed wedge with its vertex at the C o atom widening as it moves out from the atom indicates a bond with the O atom of a H subscript 2 O group. A second dashed wedge indicates a bond with the central C o atom with a B r atom up and to the right. Similarly, a solid wedge below and to the left indicates a bond with a B r atom. A second solid wedge below and to the right indicates a bond with the O atom of an H subscript 2 O group. This structure is enclosed in brackets. Outside the brackets to the right is the superscript 2 negative sign.

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 nonsuperimposable 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 superimposed. 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 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 34.3. 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.

Figure 34.3

The complex [M(en)₃]ⁿ⁺ (Mⁿ⁺ = a metal ion, en = ethylenediamine) has a nonsuperimposable mirror image.

Two structures are shown with a vertical dashed line segment between them. The structure left of this line segment has a central M representing a metal atom. To this atom, six N H subscript 2 groups are attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. The bonds to these groups are all directed toward the N atoms. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central M atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments. This structure has the overall appearance of a flower with three petals, two of which are equidistant from the dashed line. A mirror image of this structure appears on the right side of the dashed line, again with two of the “petals” equidistant from the dashed line to its left.

The [Co(en)₂Cl₂]⁺ ion exhibits geometric isomerism (cis/trans), and its cis isomer exists as a pair of optical isomers (Figure 34.4).

Figure 34.4

Three isomeric forms of [Co(en)₂Cl₂]⁺ exist. The trans isomer, formed when the chlorines are positioned at a 180° 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 figure includes three structures. The first two are labeled “cis form (optical isomers).” These structures are followed by a vertical dashed line segment to the right of which appears a third structure that is labeled “trans form.” The first structure includes a central C o atom that has four N H subscript 2 groups and two C l atoms attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. C l atoms are bonded at the top and at the upper left of the structure. The remaining four bonds extend from the central C o atom to the N atoms of N H subscript 2 groups. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central C o atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments, forming two rings in the structure. This entire structure is enclosed in brackets. Outside the brackets to the right is the superscript plus. The second structure, which appears to the be mirror image of the first structure, includes a central C o atom that has four N H subscript 2 groups and two C l atoms attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. C l atoms are bonded at the top and at the upper right of the structure. The remaining four bonds extend from the central C o atom to the N atoms of N H subscript 2 groups. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central C o atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments, forming two rings in the structure. This entire structure is enclosed in brackets. Outside the brackets to the right is a superscript plus sign. The third, trans structure includes a central C o atom that has four N H subscript 2 groups and two C l atoms attached with single bonds. These bonds are indicated with line segments extending above and below, dashed wedges extending up and to the left and right, and solid wedges extending below and to the left and right. C l atoms are bonded at the top and bottom of the structure. The remaining four bonds extend from the central C o atom to the N atoms of N H subscript 2 groups. The N H subscript 2 groups are each connected to C atoms of C H subscript 2 groups extending outward from the central C o atom. These C H subscript 2 groups are connected in pairs with bonds indicated by short line segments, forming two rings in the structure. This entire structure is enclosed in brackets. Outside the brackets to the right is a superscript plus sign. This final structure has rings of atoms on opposite sides of the structure.

Linkage isomers occur when the coordination compound contains a ligand that can bind to the transition metal center through two different atoms. For example, the CN ligand can bind through the carbon atom (cyano) or through the nitrogen atom (isocyano). Similarly, SCN− can be bound through the sulfur or nitrogen atom, affording two distinct compounds ([Co(NH₃)₅SCN]²⁺ or [Co(NH₃)₅NCS]²⁺).

Ionization isomers (or coordination isomers) occur when one anionic ligand in the inner coordination sphere is replaced with the counter ion from the outer coordination sphere. A simple example of two ionization isomers are [CoCl₆][Br] and [CoCl₅Br][Cl].

34.1.2 Isomers of Coordination Compounds

34.1.3 Coordination Complexes in Nature and Technology

Chlorophyll, the green pigment in plants, is a complex that contains magnesium (Figure 34.5). This is an example of a main group element in a coordination complex. Plants appear green because chlorophyll absorbs red and purple light; the reflected light consequently appears green. The energy resulting from the absorption of light is used in photosynthesis.

Figure 34.5

(a) Chlorophyll comes in several different forms, which all have the same basic structure around the magnesium center. (b) Copper phthalocyanine blue, a square planar copper complex, is present in some blue dyes.

Structural formulas are shown for two complex molecules. The first has a central M g atom, to which N atoms are bonded above, below, left, and right. Each N atom is a component of a 5 member ring with four C atoms. Each of these rings has a double bond between the C atoms that are not bonded to the N atom. The C atoms that are bonded to N atoms are connected to C atoms that serve as links between the 5-member rings. The bond to the C atom clockwise from the 5-member ring in each case is a double bond. The bond to the C atom counterclockwise from the 5-member ring in each case is a single bond. To the left of the structure, two of the C atoms in the 5-member rings that are not bonded to N atoms are bonded to C H subscript 3 groups. The other carbons in these rings that are not bonded to N atoms are bonded to groups above and below. A variety of groups are attached outside this interconnected system of rings, including four C H subscript 3 groups, a C H subscript 2 C H subscript 2, C O O C subscript 20, H subscript 39 group, a C H C H subscript 2 group with a double bond between the C atoms, additional branching to form a five-member carbon ring to which an O atom is double bonded and a C O O C H subscript 3 group is attached. The second structure has a central C u atom to which four N atoms that participate in 5-member rings with C atoms are bonded. Unlike the first molecule, these 5-member rings are joined by N atoms between them, with a double bond on the counter clockwise side and a single bond on the clockwise side of each of the four N atoms that link the rings. On the side of each 5-member ring opposite its N atom, four additional carbon atoms are bonded, forming 6-member carbon rings with alternating double bonds. The double bonds are not present on the bonds that are shared with the 5-member rings.

34.1.4 CHEMISTRY IN EVERYDAY LIFE

Transition Metal Catalysts

One of the most important applications of transition metals is as industrial catalysts. As you recall from the chapter on kinetics, a catalyst increases the rate of reaction by lowering the activation energy and is regenerated in the catalytic cycle. Over 90% of all manufactured products are made with the aid of one or more catalysts. The ability to bind ligands and change oxidation states makes transition metal catalysts well suited for catalytic applications. Vanadium oxide is used to produce 230,000,000 tons of sulfuric acid worldwide each year, which in turn is used to make everything from fertilizers to cans for food. Plastics are made with the aid of transition metal catalysts, along with detergents, fertilizers, paints, and more (see Figure 34.6). Very complicated pharmaceuticals are manufactured with catalysts that are selective, reacting with one specific bond out of a large number of possibilities. Catalysts allow processes to be more economical and more environmentally friendly. Developing new catalysts and better understanding of existing systems are important areas of current research.

Figure 34.6

(a) Detergents, (b) paints, and (c) fertilizers are all made using transition metal catalysts. (credit a: modification of work by “Mr. Brian”/Flickr; credit b: modification of work by Ewen Roberts; credit c: modification of work by “osseous”/Flickr)

This figure includes three photographs. In a, a photo shows store shelving filled with a variety of brands of laundry detergent. In b, a photo shows a can of yellow paint being stirred. In c, a bag of fertilizer is shown.

34.1.5 PORTRAIT OF A CHEMIST

Deanna D’Alessandro

Dr. Deanna D’Alessandro develops new metal-containing materials that demonstrate unique electronic, optical, and magnetic properties. Her research combines the fields of fundamental inorganic and physical chemistry with materials engineering. She is working on many different projects that rely on transition metals. For example, one type of compound she is developing captures carbon dioxide waste from power plants and catalytically converts it into useful products (see Figure 34.7).

Figure 34.7

Catalytic converters change carbon dioxide emissions from power plants into useful products, and, like the one shown here, are also found in cars.

An image is shown of a catalytic converter. At the upper left, a blue arrow pointing into a pipe that enters a larger, widened chamber is labeled, “Dirty emissions.” A small black arrow that points to the lower right is positioned along the upper left side of the widened region. This arrow is labeled, “Additional oxygen from air pump.” The image shows the converter with the upper surface removed, exposing a red-brown interior. The portion of the converter closes to the dirty emissions inlet shows small, spherical components in an interior layer. This layer is labeled, “Three-way reduction catalyst.” The middle region shows closely packed small brown rods that are aligned parallel to the dirty emissions inlet pipe. The final nearly quarter of the interior of the catalytic converter again shows a layer of closely packed small, red-brown circles. Two large light grey arrows extend from this layer to the open region at the lower right of the image to the label, “Clean emissions.”

Another project involves the development of porous, sponge-like materials that are “photoactive.” The absorption of light causes the pores of the sponge to change size, allowing gas diffusion to be controlled. This has many potential useful applications, from powering cars with hydrogen fuel cells to making better electronics components. Although not a complex, self-darkening sunglasses are an example of a photoactive substance.

Watch this video to learn more about this research and listen to Dr. D’Alessandro (shown in Figure 34.8) describe what it is like being a research chemist.

Figure 34.8

Dr. Deanna D’Alessandro is a functional materials researcher. Her work combines the inorganic and physical chemistry fields with engineering, working with transition metals to create new systems to power cars and convert energy (credit: image courtesy of Deanna D'Alessandro).

This is a photo of doctor Deanna D’Alessandro.

Many other coordination complexes are also brightly colored. The square planar copper(II) complex phthalocyanine blue (from Figure 34.5) is one of many complexes used as pigments or dyes. This complex is used in blue ink, blue jeans, and certain blue paints.

The structure of heme (Figure 34.9), the iron-containing complex in hemoglobin, is very similar to that in chlorophyll. In hemoglobin, the red heme complex is bonded to a large protein molecule (globin) by the attachment of the protein to the heme ligand. Oxygen molecules are transported by hemoglobin in the blood by being bound to the iron center. When the hemoglobin loses its oxygen, the color changes to a bluish red. Hemoglobin will only transport oxygen if the iron is Fe²⁺; oxidation of the iron to Fe³⁺ prevents oxygen transport.

Figure 34.9

Hemoglobin contains four protein subunits, each of which has an iron center attached to a heme ligand (shown in red), which is coordinated to a globin protein. Each subunit is shown in a different color.

A colorful model of a hemoglobin structure is shown. The molecule has four distinct quadrants that are filled with spiral, ribbon-like regions. The upper right quadrant is lavender, lower right is gold, lower left is light blue, and upper left is green. In each of these regions, clusters of approximately 25 red dots in nearly linear arrangements are present near the center.

Complexing agents often are used for water softening because they tie up such ions as Ca²⁺, Mg²⁺, and Fe²⁺, which make water hard. Many metal ions are also undesirable in food products because these ions can catalyze reactions that change the color of food. Coordination complexes are useful as preservatives. For example, the ligand EDTA, (HO₂CCH₂)₂NCH₂CH₂N(CH₂CO₂H)₂, coordinates to metal ions through six donor atoms and prevents the metals from reacting (Figure 34.10). This ligand also is used to sequester metal ions in paper production, textiles, and detergents, and has pharmaceutical uses.

Figure 34.10

The ligand EDTA binds tightly to a variety of metal ions by forming hexadentate complexes.

This structure shows a metal atom, represented by M in red. Single bonds extending from the M are also shown in red. Bonds are indicated with O atoms by line segments extending above and below. Dashed wedges extend up and to the left to an N atom and up and to the right to an O atom, and solid wedges extend below and to the left to an N atom and below and to the right to an O atom. The O atoms bonded to the M atom each have a negative sign associated with them and they are each bonded to a C atom which is in turn double bonded to an O atom and single bonded to a C atom in a C H subscript 2 group. This last C atom in each case is single bonded to one of the N atoms, resulting in two five-member rings of which the M atom is a part. To the left of each N atom, are single bonds to the C in C H subscript 2 groups, which in turn are connected with a single bond, forming another five-member ring with the two N atoms and the M atom. Extending up and to the left of the upper N atom is a bond to the C atom of another C H subscript 2 group. This group is in turn bonded to a C atom which is double bonded to an O atom and single bonded to the O atom that is bonded to the M atom at the top of the structure, again forming a five-member ring. The same bonding structure repeats at the bottom of the structure extending from the N atom bonded at the lower left of the M atom. All single bonded O atoms in this structure have negative charges associated with them.

Complexing agents that tie up metal ions are also used as drugs. British Anti-Lewisite (BAL), HSCH₂CH(SH)CH₂OH, is a drug developed during World War I as an antidote for the arsenic-based war gas Lewisite. BAL is now used to treat poisoning by heavy metals, such as arsenic, mercury, thallium, and chromium. The drug is a ligand and functions by making a water-soluble chelate of the metal; the kidneys eliminate this metal chelate (Figure 34.11). Another polydentate ligand, enterobactin, which is isolated from certain bacteria, is used to form complexes of iron and thereby to control the severe iron buildup found in patients suffering from blood diseases such as Cooley’s anemia, who require frequent transfusions. As the transfused blood breaks down, the usual metabolic processes that remove iron are overloaded, and excess iron can build up to fatal levels. Enterobactin forms a water-soluble complex with excess iron, and the body can safely eliminate this complex.

Figure 34.11

Coordination complexes are used as drugs. (a) British Anti-Lewisite is used to treat heavy metal poisoning by coordinating metals (M), and enterobactin (b) allows excess iron in the blood to be removed.

This figure includes two structures. In a, a five member ring is shown with an S atom at the top with additional atoms single bonded in the following order clockwise around the pentagonal ring; M atom, S atom, C atom of a C H subscript 2 group, followed by a C atom of a C H group. The final C atom is bonded to the original S atom completing the ring. The C in the C H group is at the upper left of the structure. This C has a C H subscript 2 group bonded above to which an O H group is bonded to the right. In b, a complex structure is shown. It has an open central region and multiple ring structures. A single F e atom is included, appearing to be bonded to six O atoms. Fifteen total O atoms are bonded into the structure along with three N atoms and multiple C atoms and H atoms. Nine O atoms are single bonded and are incorporated into rings and six are double bonded, extending outward from ring structures.

EXAMPLE 34.1.6

Chelation Therapy

Ligands like BAL and enterobactin are important in medical treatments for heavy metal poisoning. However, chelation therapies can disrupt the normal concentration of ions in the body, leading to serious side effects, so researchers are searching for new chelation drugs. One drug that has been developed is dimercaptosuccinic acid (DMSA), shown in Figure 34.12. Identify which atoms in this molecule could act as donor atoms.

Figure 34.12

Dimercaptosuccinic acid is used to treat heavy metal poisoning.

A structure is shown that has an H atom on the far left which is single bonded to an O atom to its right. This atom is bonded to a C atom just below and to the right. This C atom has a double bonded O atom below and is bonded to the C atom of a C H group. Above this C atom, a solid wedge extends upward to the S atom of an S H group. A bond extends from this last C atom to another C atom of a second C H group below and to the right. A dashed wedge extends from this C atom to the S of an S H group below. A single bond extends up and to the right of the C atom to another C atom. This last C atom has a double bonded O atom above. A single bond extends to a second O atom below and to the right. To the right of this O atom, an H atom is connected with a single bond. All S and O atoms in the structure are shown with two unshared pairs of electron dots.

Solution

All of the oxygen and sulfur atoms have lone pairs of electrons that can be used to coordinate to a metal center, so there are six possible donor atoms. Geometrically, only two of these atoms can be coordinated to a metal at once. The most common binding mode involves the coordination of one sulfur atom and one oxygen atom, forming a five-member ring with the metal.

Check Your Learning

Some alternative medicine practitioners recommend chelation treatments for ailments that are not clearly related to heavy metals, such as cancer and autism, although the practice is discouraged by many scientific organizations.¹ Identify at least two biologically important metals that could be disrupted by chelation therapy.

Answer

Ca, Fe, Zn, and Cu

Ligands are also used in the electroplating industry. When metal ions are reduced to produce thin metal coatings, metals can clump together to form clusters and nanoparticles. When metal coordination complexes are used, the ligands keep the metal atoms isolated from each other. It has been found that many metals plate out as a smoother, more uniform, better-looking, and more adherent surface when plated from a bath containing the metal as a complex ion. Thus, complexes such as [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ are used extensively in the electroplating industry.

In 1965, scientists at Michigan State University discovered that there was a platinum complex that inhibited cell division in certain microorganisms. Later work showed that the complex was cis-diamminedichloroplatinum(II), [Pt(NH₃)₂(Cl)₂], and that the trans isomer was not effective. The inhibition of cell division indicated that this square planar compound could be an anticancer agent. In 1978, the US Food and Drug Administration approved this compound, known as cisplatin, for use in the treatment of certain forms of cancer. Since that time, many similar platinum compounds have been developed for the treatment of cancer. In all cases, these are the cis isomers and never the trans isomers. The diammine (NH₃)₂ portion is retained with other groups, replacing the dichloro [(Cl)₂] portion. The newer drugs include carboplatin, oxaliplatin, and satraplatin.

Footnotes

1National Council against Health Fraud, NCAHF Policy Statement on Chelation Therapy, (Peabody, MA, 2002).

Previous Citation(s)

Flowers, P., et al. (2019). Chemistry: Atoms First 2e. https://openstax.org/details/books/chemistry-atoms-first-2e (19.2)

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