What is a coordination compound?

A coordination compound is a substance with a chemical structure in which a central metal atom is surrounded by nonmetal atoms or groups of atoms, known as ligands, connected via chemical bonds. These compounds can be composed of electrically neutral molecules or charged species (ions). Coordination compounds are characterized by the central metal atom or ion, its oxidation state, and the number, type, and arrangement of ligands. The metal atom can be neutral or charged, while ligands can be neutral molecules, negatively charged ions (anions), or positively charged ions (cations). Coordination compounds have various applications, including serving as catalysts to alter the rate of chemical reactions. They are also studied to understand molecular structure and chemical bonding and because of their unusual chemical nature and useful properties.

What are the main parts of a coordination compound?

Coordination compounds, also known as complexes, have a structure in which a central metal atom is bonded to surrounding nonmetal atoms or groups of atoms, called ligands , through chemical bonds. These ligands can be neutral molecules or charged ions (anions or cations). Coordination compounds are characterized by: * The central metal atom or ion * The oxidation state of the metal atom or ion * The number, type, and arrangement of ligands The central metal atom or ion may be neutral or charged. Ligands can attach through one atom (monodentate) or several (polydentate). Polydentate ligands form cyclic complexes called chelates . The coordination number indicates the number of bonds from the ligands to the metal atom, typically ranging from 2 to 12, with 4 and 6 being most common.

What is a ligand in a coordination compound?

In chemistry, a ligand is an atom, ion, or molecule that binds to a central metal atom in a coordination compound . Ligands act as electron-pair donors, forming a coordinate covalent bond with the metal atom, which acts as an electron-pair acceptor, in what is known as a Lewis acid-base reaction. Ligands can be neutral molecules like water ((H_2O)), ammonia ((NH_3)), or carbon monoxide (CO), or anions like cyanide ((CN^-)), chloride ((Cl^-)), and hydroxide ((OH^-)). They can be monodentate, attaching through a single atom, or polydentate, attaching through multiple atoms. Polydentate ligands create cyclic complexes called chelates, known for their stability and utility.

How do coordination compounds form?

Coordination compounds form through coordinate bonds between a central metal atom or ion and surrounding ligands. These ligands, which can be neutral molecules or ions, act as electron-pair donors (Lewis bases), while the metal center acts as an electron-pair acceptor (Lewis acid). Werner suggested that central atoms can form coordination compounds because they can form secondary, or coordinate, bonds, in addition to valence bonds. The number and type of ligands, the metal's oxidation state, and their arrangement characterize these compounds. Virtually all metallic elements can form coordination compounds.

What are some examples of coordination compounds found in everyday life?

Coordination compounds are substances with a central metal atom bonded to surrounding atoms or groups of atoms, called ligands. They are vital in living organisms and have varied applications in chemistry and technology. Examples of coordination compounds: * Metalloenzymes Many enzymes, naturally occurring catalysts that regulate biological processes, are metal complexes. Carboxypeptidase, which aids in digestion, contains a zinc ion, while catalase, which decomposes hydrogen peroxide, contains iron-porphyrin complexes. * Hemoglobin The iron-porphyrin complexes in hemoglobin enable it to act as an oxygen carrier. * Chlorophyll This magnesium-porphyrin complex is another biologically important coordination compound. * Vitamin B12 Vitamin B12 is a complex of cobalt with a macrocyclic ligand known as corrin. * EDTA Ethylenediaminetetraacetic acid (EDTA) is a complexing agent widely used to soften water and preserve organic substances. * Dyes and Pigments Many coordination compounds have brilliant and intense colors, making them valuable as dyes and pigments . Phthalocyanine complexes, such as copper phthalocyanine, are an important class of dyes for fabrics.

How are coordination compounds important in biology and medicine?

Coordination compounds are crucial in both biology and medicine. In biological systems, metal complexes are present in many enzymes that act as catalysts. For instance, carboxypeptidase , which aids in digestion, contains a zinc ion, and catalase , which facilitates hydrogen peroxide decomposition, contains iron-porphyrin complexes. Hemoglobin , chlorophyll , and vitamin B12 are other biologically significant coordination compounds. Coordination compounds are also used as catalysts to alter the rate of chemical reactions. Complex metal catalysts are important in the production of polyethylene and polypropylene .

coordination compound

Introduction
Coordination compounds in nature
Coordination compounds in industry
History of coordination compounds
Characteristics of coordination compounds
- Coordination number
- Ligands and chelates
  - Mononuclear, monodentate
  - Polydentate
  - Polynuclear
- Nomenclature
Structure and bonding of coordination compounds
- Geometry
- Isomerism
  - Cis-trans isomerism
  - Enantiomers and diastereomers
  - Ionization isomerism
  - Linkage isomerism
  - Coordination isomerism
  - Ligand isomerism
- Bonding theories
  - Valence bond theory
  - Crystal field theory
  - Ligand field and molecular orbital theories
Principal types of complexes
- Aqua complexes
- Halo complexes
- Carbonyl complexes
- Nitrosyl complexes
- Cyano and isocyano complexes
- Organometallic complexes
- Isopoly and heteropoly anions
Important types of reactions of coordination compounds
- Acid-base
- Substitution
- Lability and inertness
- Isomerization
- Oxidation-reduction
Synthesis of coordination compounds

References & Edit History Quick Facts & Related Topics

Images

Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups.

A periodic table diagram highlights the s, p, d, and f blocks, with most elements shaded to indicate the metals relevant to organometallic chemistry. Group numbers are labeled at the top.

Diagram of the hemoglobin tetramer showing two alpha and two beta subunits, each in a different color, with four heme groups labeled, each containing a central iron atom for oxygen binding.

Diagram showing the bonding of a carbon monoxide (CO) ligand to a metal atom in a coordination compound. Image A illustrates sigma bond formation with orbital overlap, while image B shows pi backbonding between metal and CO, with labeled orbitals indicating electron flow.

A chemical diagram of tetracarbonylnickel, Ni(CO)4, shows a central nickel atom bonded to four carbonyl groups in a tetrahedral arrangement, alongside a geometric representation of the tetrahedral structure.

At a Glance

coordination compound summary

coordination compound

chemistry

Also known as: complex compound, coordinate compound, coordination complex(Show More)

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Top Questions

What is a coordination compound?
A coordination compound is a substance with a chemical structure in which a central metal atom is surrounded by nonmetal atoms or groups of atoms, known as ligands, connected via chemical bonds. These compounds can be composed of electrically neutral molecules or charged species (ions).

Coordination compounds are characterized by the central metal atom or ion, its oxidation state, and the number, type, and arrangement of ligands. The metal atom can be neutral or charged, while ligands can be neutral molecules, negatively charged ions (anions), or positively charged ions (cations).

Coordination compounds have various applications, including serving as catalysts to alter the rate of chemical reactions. They are also studied to understand molecular structure and chemical bonding and because of their unusual chemical nature and useful properties.
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What are the main parts of a coordination compound?
Coordination compounds, also known as complexes, have a structure in which a central metal atom is bonded to surrounding nonmetal atoms or groups of atoms, called ligands, through chemical bonds. These ligands can be neutral molecules or charged ions (anions or cations).

Coordination compounds are characterized by:
* The central metal atom or ion
* The oxidation state of the metal atom or ion
* The number, type, and arrangement of ligands

The central metal atom or ion may be neutral or charged. Ligands can attach through one atom (monodentate) or several (polydentate). Polydentate ligands form cyclic complexes called chelates. The coordination number indicates the number of bonds from the ligands to the metal atom, typically ranging from 2 to 12, with 4 and 6 being most common.
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What is a ligand in a coordination compound?
In chemistry, a ligand is an atom, ion, or molecule that binds to a central metal atom in a coordination compound. Ligands act as electron-pair donors, forming a coordinate covalent bond with the metal atom, which acts as an electron-pair acceptor, in what is known as a Lewis acid-base reaction.

Ligands can be neutral molecules like water ((H_2O)), ammonia ((NH_3)), or carbon monoxide (CO), or anions like cyanide ((CN^-)), chloride ((Cl^-)), and hydroxide ((OH^-)). They can be monodentate, attaching through a single atom, or polydentate, attaching through multiple atoms. Polydentate ligands create cyclic complexes called chelates, known for their stability and utility.
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How do coordination compounds form?
Coordination compounds form through coordinate bonds between a central metal atom or ion and surrounding ligands. These ligands, which can be neutral molecules or ions, act as electron-pair donors (Lewis bases), while the metal center acts as an electron-pair acceptor (Lewis acid).

Werner suggested that central atoms can form coordination compounds because they can form secondary, or coordinate, bonds, in addition to valence bonds. The number and type of ligands, the metal's oxidation state, and their arrangement characterize these compounds. Virtually all metallic elements can form coordination compounds.
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What are some examples of coordination compounds found in everyday life?
Coordination compounds are substances with a central metal atom bonded to surrounding atoms or groups of atoms, called ligands. They are vital in living organisms and have varied applications in chemistry and technology.

Examples of coordination compounds:
* Metalloenzymes Many enzymes, naturally occurring catalysts that regulate biological processes, are metal complexes. Carboxypeptidase, which aids in digestion, contains a zinc ion, while catalase, which decomposes hydrogen peroxide, contains iron-porphyrin complexes.
* Hemoglobin The iron-porphyrin complexes in hemoglobin enable it to act as an oxygen carrier.
* Chlorophyll This magnesium-porphyrin complex is another biologically important coordination compound.
* Vitamin B12 Vitamin B12 is a complex of cobalt with a macrocyclic ligand known as corrin.
* EDTA Ethylenediaminetetraacetic acid (EDTA) is a complexing agent widely used to soften water and preserve organic substances.
* Dyes and Pigments Many coordination compounds have brilliant and intense colors, making them valuable as dyes and pigments. Phthalocyanine complexes, such as copper phthalocyanine, are an important class of dyes for fabrics.
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How are coordination compounds important in biology and medicine?
Coordination compounds are crucial in both biology and medicine. In biological systems, metal complexes are present in many enzymes that act as catalysts. For instance, carboxypeptidase, which aids in digestion, contains a zinc ion, and catalase, which facilitates hydrogen peroxide decomposition, contains iron-porphyrin complexes. Hemoglobin, chlorophyll, and vitamin B12 are other biologically significant coordination compounds.

Coordination compounds are also used as catalysts to alter the rate of chemical reactions. Complex metal catalysts are important in the production of polyethylene and polypropylene.
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coordination compound, any of a class of substances with chemical structures in which a central metal atom is surrounded by nonmetal atoms or groups of atoms, called ligands, joined to it by chemical bonds. Coordination compounds include such substances as vitamin B₁₂, hemoglobin, and chlorophyll, dyes and pigments, and catalysts used in preparing organic substances.

A major application of coordination compounds is their use as catalysts, which serve to alter the rate of chemical reactions. Certain complex metal catalysts, for example, play a key role in the production of polyethylene and polypropylene. In addition, a very stable class of organometallic coordination compounds has provided impetus to the development of organometallic chemistry. Organometallic coordination compounds are sometimes characterized by “sandwich” structures, in which two molecules of an unsaturated cyclic hydrocarbon, which lacks one or more hydrogen atoms, bond on either side of a metal atom. This results in a highly stable aromatic system.

The following article covers the history, applications, and characteristics (including structure and bonding, principle types of complexes, and reactions and syntheses) of coordination compounds. For more information about specific properties or types of coordination compounds, see the articles isomerism; coordination number; chemical reaction; and organometallic compound.

Coordination compounds in nature

Naturally occurring coordination compounds are vital to living organisms. Metal complexes play a variety of important roles in biological systems. Many enzymes, the naturally occurring catalysts that regulate biological processes, are metal complexes (metalloenzymes); for example, carboxypeptidase, a hydrolytic enzyme important in digestion, contains a zinc ion coordinated to several amino acid residues of the protein. Another enzyme, catalase, which is an efficient catalyst for the decomposition of hydrogen peroxide, contains iron-porphyrin complexes. In both cases, the coordinated metal ions are probably the sites of catalytic activity. Hemoglobin also contains iron-porphyrin complexes, its role as an oxygen carrier being related to the ability of the iron atoms to coordinate oxygen molecules reversibly. Other biologically important coordination compounds include chlorophyll (a magnesium-porphyrin complex) and vitamin B₁₂, a complex of cobalt with a macrocyclic ligand known as corrin.

Coordination compounds in industry

The applications of coordination compounds in chemistry and technology are many and varied. The brilliant and intense colors of many coordination compounds, such as Prussian blue, render them of great value as dyes and pigments. Phthalocyanine complexes (e.g., copper phthalocyanine), containing large-ring ligands closely related to the porphyrins, constitute an important class of dyes for fabrics.

Several important hydrometallurgical processes utilize metal complexes. Nickel, cobalt, and copper can be extracted from their ores as ammine complexes using aqueous ammonia. Differences in the stabilities and solubilities of the ammine complexes can be utilized in selective precipitation procedures that bring about separation of the metals. The purification of nickel can be effected by reaction with carbon monoxide to form the volatile tetracarbonylnickel complex, which can be distilled and thermally decomposed to deposit the pure metal. Aqueous cyanide solutions usually are employed to separate gold from its ores in the form of the extremely stable dicyanoaurate(−1) complex. Cyanide complexes also find application in electroplating.

There are a number of ways in which coordination compounds are used in the analysis of various substances. These include (1) the selective precipitation of metal ions as complexes—for example, nickel(2+) ion as the dimethylglyoxime complex (shown below), Coordination Compound: the selective precipitation of nicke(II) ion as the dimethylglyoxime complex (2) the formation of colored complexes, such as the tetrachlorocobaltate(2−) ion, which can be determined spectrophotometrically—that is, by means of their light absorption properties, and (3) the preparation of complexes, such as metal acetylacetonates, which can be separated from aqueous solution by extraction with organic solvents.

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In certain circumstances, the presence of metal ions is undesirable, as, for example, in water, in which calcium (Ca²⁺) and magnesium (Mg²⁺) ions cause hardness. In such cases the undesirable effects of the metal ions frequently can be eliminated by “sequestering” the ions as harmless complexes through the addition of an appropriate complexing reagent. Ethylenediaminetetraacetic acid (EDTA) forms very stable complexes, and it is widely used for this purpose. Its applications include water softening (by tying up Ca²⁺ and Mg²⁺) and the preservation of organic substances, such as vegetable oils and rubber, in which case it combines with traces of transition metal ions that would catalyze oxidation of the organic substances.

A technological and scientific development of major significance was the discovery in 1954 that certain complex metal catalysts—namely, a combination of titanium trichloride, or TiCl₃, and triethylaluminum, or Al(C₂H₅)₃—bring about the polymerizations of organic compounds with carbon-carbon double bonds under mild conditions to form polymers of high molecular weight and highly ordered (stereoregular) structures. Certain of these polymers are of great commercial importance because they are used to make many kinds of fibers, films, and plastics. Other technologically important processes based on metal complex catalysts include the catalysis by metal carbonyls, such as hydridotetracarbonylcobalt, of the so-called hydroformylation of olefins—i.e., of their reactions with hydrogen and carbon monoxide to form aldehydes—and the catalysis by tetrachloropalladate(2−) ions of the oxidation of ethylene in aqueous solution to acetaldehyde (see chemical reaction and catalysis).

coordination compound

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Coordination compounds in nature

Coordination compounds in industry