Ligand
Ligand
Metal-ligand bonds in biological chemistry
In inorganic chemistry, ligands are molecules or electrically charged atoms (ions) that are bonded to metal atoms or ions. The ligand changes the metal’s ability to dissolve in or react with its surroundings. In biochemistry, ligands are defined as molecules, usually protein, that change the biological activity of other molecules by bonding with them. The inorganic meaning is more common, and will be the subject of this article.
Structure and bonding
The bonding atoms of ligands are usually nonmetal elements such as oxygen, nitrogen, or chlorine. Whether alone or in molecules such as water or ammonia, these atoms have pairs of electrons that are not involved in chemical bonds. The electron pairs can enter the space around the metal atom and bond with it (Figure 1).
Thus, the metal and ligand are joined by a covalent bond, consisting of two electrons shared between them. However, both electrons are provided by the ligand itself (see Figure 2).
Metal atoms or ions usually bond to two, four, or six ligands. These are arranged with geometric symmetry around the central metal atom. The metal together with its ligands are called a coordination compound. If the structure has an overall electrical charge it is called a complex ion.
Because of their shapes, there can be different coordination compounds having exactly the same atoms and bonds, but arranged differently. Such molecules are called geometric isomers (Figure 3).
The different arrangement causes differences in physical properties such as color and melting temperature. Geometric isomers also differ in their chemical reactions, especially when these occur in living organisms. That is because the molecules that make up living things are themselves usually geometric isomers with very specific shapes. Reactions only occur between molecules whose shapes match each other, like a key fitted to a lock.
An example of a biologically active geometric isomer is cisplatin, a coordination compound used in medicine to suppress tumors. The molecule consists of a platinum atom surrounded by two ammonia molecules and two chlorine atoms. The four ligands lie at the corners of a square, with ligands of the same kind as neighbors.
The isomer transplatin, in which they are diagonally opposite each other, has no affect on tumors.
Compounds of metals with ligands are often brightly colored. This results from repulsion between the electrons of the ligand and those of the metal atom itself. The atom’s electrons are also geometrically arranged around its nucleus. They occupy regions called orbitals. In an isolated metal atom, groups of similar orbitals have the same energy. But when ligands bond to the atom, they approach some orbitals more closely than others. Electrons in the closer orbital are repelled more strongly. They must have more energy to occupy those orbitals. The energy difference is called “crystal field splitting” (see Figure 4).
The metal’s electrons can move to a higher energy orbital by absorbing energy from visible light. Therefore, such compounds appear colored.
Chelating agents
Some ligands can form more than one bond to a single metal atom. These are called chelating agents. The name comes from the Greek word chele, meaning “claw.” The ligands surround the metal atom and hold it as if in a claw. Because they hold metals so strongly, chelates are also referred to as “metal scavengers.” They effectively remove metal atoms and prevent them from reacting with anything else.
One of the best-known chelating agents is ethylenediaminetetraacetic acid, or EDTA. It contains 32 atoms, six of which can bond to a single metal atom (Figure 5).
EDTA is a common food preservative. Foods contain ions of iron, zinc, magnesium, and other metals. These are natural components of food substances, but they hasten the chemical reactions that cause flavor and color to deteriorate. EDTA added to foods forms strong, stable bonds to the metal ions, blocking their chemical activity. EDTA is also used to treat lead poisoning in human beings. The EDTA-lead complex is safely excreted in body waste.
Metal-ligand bonds in biological chemistry
The porphyrin ring is a chelate that plays several different roles in the chemistry of living things (Figure 6).
Bound to magnesium, it forms chlorophyll, the green pigment which is central to photosynthesis in plants. Bound to iron, it forms cytochrome molecules, which help transfer energy throughout living cells. Hemoglobin, which gives blood its red color and carries oxygen to body cells, is also an iron- porphyrin molecule. Iron can form six bonds to ligands, and the porphyrin ring uses only four bonds. Of the remaining two, one holds a protein molecule, and the other holds the oxygen molecule which will be delivered to the cells.
KEY TERMS
Coordination compound —A molecule consisting of a metal atom and the ligands to which it is bound.
Covalent bond —A chemical bond formed when two atoms share a pair of electrons with each other.
Ion —An atom or molecule which has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).
Isomers —Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.
Orbital —A region of space around an atomic nucleus likely to be occupied by an electron.
Other uses
Inorganic salts, which contain metal ions, do not dissolve in organic solvents such as benzene. However, by surrounding the metal ion with a chelate called a “crown ether” the desired solution can be made. The mining industry uses cyanide ions to dissolve gold out of the quartz rocks in which it is often found. The cyanide ligands are removed in subsequent chemical steps.
Resources
BOOKS
Newton, David E. Consumer Chemistry Projects for Young Scientists. New York: Franklin Watts, 1991.
PERIODICALS
Schmidt, Karen F. “Mirror-Image Molecules.” Science News 143 (29 May 1993): 348-350.
OTHER
“Molecular Architecture.” Unit 9 of The World of Chemistry. Videotape Series. University of Maryland at College Park. Gilbert Castellan, Nava Ben-Zvi and Isidore Adler, project co-directors. The Annenberg/ CPB Project, 1990.
University of Oxford, Department of Chemistry “Special Topics in Organometallic Chemistry: Metal–Ligand Multiple Bonds” < http://www.chem.ox.ac.uk/it_lectures/organometalchem/MLmultiplebonds/lecture1/index.html> (accessed December 2, 2006).
Sara G. B. Fishman
Ligand
Ligand
In inorganic chemistry , ligands are molecules or electrically charged atoms (ions) which are bonded to metal atoms or ions. The ligand changes the metal's ability to dissolve in or react with its surroundings. In biochemistry , ligands are defined as molecules, usually
protein, that change the biological activity of other molecules by bonding with them. The inorganic meaning is more common, and will be the subject of this article.
Structure and bonding
The bonding atoms of ligands are usually non-metal elements such as oxygen , nitrogen , or chlorine . Whether alone or in molecules such as water or ammonia , these atoms have pairs of electrons that are not involved in chemical bonds. The electron pairs can enter the space around the metal atom and bond with it.
Thus, the metal and ligand are joined by a covalent bond, consisting of two electrons shared between them. However, both electrons are provided by the ligand itself.
Metal atoms or ions usually bond to two, four or six ligands. These are arranged with geometric symmetry around the central metal atom. The metal together with its ligands are called a coordination compound . If the structure has an overall electrical charge it is called a complex ion.
Because of their shapes, there can be different coordination compounds having exactly the same atoms and bonds, but arranged differently. Such molecules are called geometric isomers.
The different arrangement causes differences in physical properties such as color and melting temperature . Geometric isomers also differ in their chemical reactions , especially when these occur in living organisms. That is because the molecules which make up living things are themselves usually geometric isomers with very specific shapes. Reactions only occur between molecules whose shapes match each other, like a key fitted to a lock.
An example of a biologically active geometric isomer is cisplatin, a coordination compound used in medicine to suppress tumors. The molecule consists of a platinum atom surrounded by two ammonia molecules and two chlorine atoms. The four ligands lie at the corners of a square , with ligands of the same kind as neighbors.
The isomer transplatin, in which they are diagonally opposite each other, has no affect on tumors.
Compounds of metals with ligands are often brightly colored. This results from repulsion between the electrons of the ligand and those of the metal atom itself. The atom's electrons are also geometrically arranged around its nucleus. They occupy regions called orbitals. In an isolated metal atom, groups of similar orbitals have the same energy . But when ligands bond to the atom, they approach some orbitals more closely than others. Electrons in the closer orbital are repelled more strongly. They must have more energy
to occupy those orbitals. The energy difference is called "crystal field splitting."
The metal's electrons can move to a higher energy orbital by absorbing energy from visible light . Therefore, such compounds appear colored.
Chelating agents
Some ligands can form more than one bond to a single metal atom. These are called chelating agents. The name comes from the Greek word chele, meaning "claw." The ligands surround the metal atom and hold it as if in a claw. Because they hold metals so strongly, chelates are also referred to as "metal scavengers." They effectively remove metal atoms and prevent them from reacting with anything else.
One of the best-known chelating agents is ethylene-diaminetetraacetic acid, or EDTA. It contains 32 atoms, six of which can bond to a single metal atom.
EDTA is a common food preservative. Foods contain ions of iron , zinc, magnesium , and other metals. These are natural components of food substances, but they hasten the chemical reactions which cause flavor and color to deteriorate. EDTA added to foods forms strong, stable bonds to the metal ions, blocking their chemical activity. EDTA is also used to treat lead poisoning in human beings. The EDTA-lead complex is safely excreted in body waste.
Metal-ligand bonds in biological chemistry
The porphyrin ring is a chelate that plays several different roles in the chemistry of living things.
Bound to magnesium, it forms chlorophyll , the green pigment which is central to photosynthesis in plants. Bound to iron, it forms cytochrome molecules, which assist in the transfer of energy throughout living cells. Hemoglobin, which gives blood its red color and carries oxygen to body cells, is also an iron-porphyrin molecule. Iron can form six bonds to ligands, and the porphyrin ring uses only four bonds. Of the remaining two, one holds a protein molecule, and the other holds the oxygen molecule which will be delivered to the cells.
Other uses
Inorganic salts, which contain metal ions, do not dissolve in organic solvents such as benzene . However, by surrounding the metal ion with a chelate called a "crown ether" the desired solution can be made. The mining industry uses cyanide ions to dissolve gold out of the quartz rocks in which it is often found. The cyanide ligands are removed in subsequent chemical steps.
Resources
books
Benarde, Melvin A. The Chemicals We Eat. New York: American Heritage Press, 1971.
Newton, David E. Consumer Chemistry Projects for Young Scientists. New York: Franklin Watts, 1991.
periodicals
Schmidt, Karen F. "Mirror-Image Molecules." Science News 143 (May 29, 1993): 348-350.
other
"Molecular Architecture." Unit 9 of The World of Chemistry. Videotape Series. University of Maryland at College Park. Gilbert Castellan, Nava Ben-Zvi, and Isidore Adler, project co-directors. The Annenberg/CPB Project, 1990.
Sara G. B. Fishman
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Coordination compound
—A molecule consisting of a metal atom and the ligands to which it is bound.
- Covalent bond
—A chemical bond formed when two atoms share a pair of electrons with each other.
- Ion
—An atom or molecule which has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).
- Isomers
—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.
- Orbital
—A region of space around an atomic nucleus likely to be occupied by an electron.
ligand
1. An atom, ion, or molecule that acts as the electron-donor partner in one or more co-ordination bonds. A heterocyclic ring is formed if the ligand is an organic compound, and the product is termed a chelate.
2. A molecule (e.g. an antibody) that can bind to specific sites on cell membranes.
ligand
ligand
1. (in chemistry) An ion, atom, or molecule that donates a pair of electrons to a metal atom to form a type of covalent bond called a coordinate bond.
2. (in cell biology) A molecule that binds to a protein with a high degree of specificity. Examples are the substrate of an enzyme and a hormone binding to a cell receptor.