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Coordination Complexes in Nature and Technology

Coordination Complexes in Nature and Technology

Chlorophyll, the green pigment in plants, is a complex that contains magnesium (see the figure below). 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.

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.

(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.

Many other coordination complexes are also brightly colored. The square planar copper(II) complex phthalocyanine blue (from the figure above) 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 (see the figure below), 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.

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.

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.

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 (see the figure below). This ligand also is used to sequester metal ions in paper production, textiles, and detergents, and has pharmaceutical uses.

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.

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

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 (see the figure below).

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.

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.

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.

Example

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 the figure below. Identify which atoms in this molecule could act as donor atoms.

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.

Dimercaptosuccinic acid is used to treat heavy metal poisoning.

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.

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.

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