Transition Elements

Type of physical science: Chemistry

Field of study: Chemistry of the elements

Transition elements are a group of twenty-seven chemical elements with similarities in chemical reactivity and electronic structure. They are widely used in chemical processes and are components of several important biomolecules.

Overview

Transition elements are the largest subgroup of chemical elements. They are characterized by similarities in electronic structure and chemical reactivity that distinguish them from non-transition elements. Without exception, the transition elements are metals. Compared to metals that are not transition elements, such as lead or tin, transition elements are hard and strong, possess a high luster, typically have very high boiling and melting points, and are excellent conductors of both heat and electricity. In addition, transition elements readily form alloys with other metals, both transition and non-transition. Most transition elements dissolve readily in mineral acids, with the exception of noble metals such as silver, gold, or platinum.

Additional properties of the transition elements clearly distinguish them from other types of elements. All metallic elements form positive ions, or cations, in compounds via loss of electrons from the metal atom, resulting in a positive electrical charge on the metal atom.

Nontransition metals typically display only one stable electrical charge, or oxidation state. For the transition elements, without exception, there is more than one stable oxidation state possible.

Compounds containing transition metal ions display interesting magnetic properties and are often colored, with color typically dependent on the metal and its oxidation state.

Iron is the most abundant transition element. Its properties are typical of transition metals, as iron displays two stable oxidation states in its compounds. Iron is also found in brightly colored compounds such as hemoglobin. The red and blue of oxygenated and deoxygenated blood are caused by the presence of iron in hemoglobin.

To understand the chemical properties of the transition elements, an understanding of their electronic structure is necessary. In a simple picture of the atom, the nucleus is surrounded by a series of electron-containing shells at increasing distances from the nucleus. The farther a shell is from the nucleus, the higher the energy of electrons placed in that shell. Within a shell, there exist one or more subshells, which are sets of atomic orbitals. Electrons occupy these orbitals, which describe the region of space in which a particular electron is localized. Atomic orbitals within a subshell have identical energies and shapes, differing from one another only by their orientation in space. Subshells within a shell differ slightly in energy, so that in actuality a shell includes a range of orbital energies.

To understand how electrons in an atom occupy the available orbitals, a concept called the aufbau principle must be employed. Aufbau means, in German, "to build up." In the aufbau process, the electronic arrangement, or configuration, of an atom is described by hypothetically placing electrons one at a time in available orbitals so that each electron is placed in the lowest-energy orbital available on the atom. Ideally, in so doing, one shell would be completely filled with electrons before any were placed in the next, higher energy shell. It is found, however, that shells become less separated farther from the nucleus, and overlapping of different shells can occur. For instance, the highest energy subshell in the third shell, called a d subshell, is at a slightly higher energy than the lowest energy subshell in the fourth shell.

Overlapping of shells is critical to understanding the electronic arrangement of the transition elements. In the aufbau process for transition elements, this arrangement is characterized by the placement of two electrons in the outermost, or valence, shell, followed by placement of the remaining electrons in an inner d subshell containing five orbitals. A maximum of ten electrons can be placed in this subshell, as each atomic orbital can contain a maximum of two.

There are three sets of elements in the periodic table that display this type of configuration. The transition elements are those in which this inner d subshell is partially filled. In the periodic table, the transition, or d-block, elements are in the center and include the three rows from scandium to copper, yttrium to silver, and lanthanum to gold.

The unique properties of the transition metals are explained by their typical electronic configuration. It is easy to remove the two electrons chemically from the outermost shell. As a result, virtually all transition elements have a stable oxidation state corresponding to the loss of these electrons. Additional stable oxidation states correspond to loss of electrons from the inner d subshell. The partially filled nature of this subshell also explains the magnetic properties of the transition elements.

An electron, which is an electrically charged particle, behaves as though it were spinning about an axis, which generates a magnetic field. There are two possible directions of electron spin, which generate opposing magnetic fields. An orbital can contain a maximum of two electrons with opposite, or paired, spins. When all the electrons in an atom are spin paired, the magnetic fields generated by the electrons are all negated. If one or more electrons are not paired with another electron of opposite spin, however, the atom will produce a net magnetic field and thus be attracted to a magnet. Transition elements form a class of compounds called coordination compounds, in which a diversity of magnetic behavior may be displayed by the same element in different compounds.

Coordination complexes are formed when several chemical species, called ligands, form coordinate covalent bonds with the metal. Covalent bonds are bonds in which an electron pair is shared between two atoms. In a coordinate bond, one atom donates both of the shared electrons to the bond. In a complex, the ligands are arranged about a central metal ion in such a way that the ligands maximize their separation. As a result, the metal is surrounded by several ligands that occupy the corners of an imaginary polyhedron about the metal. These geometries are characteristic of certain numbers of ligands, or coordination numbers. The most common coordination numbers are six and four, and corresponding geometries are octahedral and tetrahedral, respectively.

Complexes often absorb visible light. When exposed to white light, they selectively remove one color so that the reaming light appears to have the complementary color. For a given metal, the frequency of light absorbed depends on the type of ligand present. The chloride ion and water, as ligands, form complexes that absorb light of lower frequency than cyanide or carbon monoxide complexes of the same metal. Coupled with this trend is the magnetic behavior of complexes. High-spin complexes are complexes that have the same number of unpaired electrons as the free transition metal ion. Low-spin complexes contain fewer unpaired electrons.

Chloride and water complexes tend to form high-spin complexes, while cyanide and carbon monoxide complexes are low-spin.

Two theories of bonding in coordination complexes are used to explain these observations. Crystal field theory is an electrostatic model; its main postulate is that the ligands cause the orbitals in the partially filled d subshell to split into different energy levels. The resulting pattern of d orbitals depends on the geometry of the complex, and different ligands cause different degrees of splitting. Strong field ligands cause large splitting, while weak field ligands cause slight splitting. Complexes of strong field ligands tend to be low-spin as a result of the great energy gap caused by large splitting of the d orbitals. When electrons in the lower subset of split orbitals absorb light of the correct frequency, they absorb energy and jump to the higher orbital subset. The greater the splitting, the higher the frequency of light required to cause this transition. Thus, the difference in color of complexes caused by ligands can be explained in terms of the degree of splitting caused by the ligand. Ligands can be ranked according to increasing crystal field splitting. Such a listing is called a spectrochemical series and may be used to make predictions about the properties of different complexes.

Molecular orbital theory is another theory of bonding in complexes, in which the atomic orbitals of the metal and ligands combine to form a new set of molecular orbitals in the complex. This is the more accurate picture of bonding, but the results obtained in this theory are essentially the same as those of crystal field theory.

Applications

Gold, silver, and platinum are three transition metals almost universally recognized.

Throughout history, these precious metals have been valued for their rarity and beauty. This is reflected in the use of these elements as jewelry, currency, and backing for paper currency. Gold and silver are also excellent conductors of electricity, more so than copper at high temperatures, and are often used in electrical components and wiring that will be subjected to such an extreme.

Historically, copper and iron have been the most useful of the transition elements.

Bronze, an alloy of copper and tin, and steel, an alloy of iron and carbon, are vitally important in the history of civilization. Steel is indispensable to modern society because of its strength and availability.

Many of the transition elements that are not highly abundant in the earth's crust are still vital to modern metallurgy. Titanium is produced in large quantities and utilized in the aircraft industry, primarily because it is an extremely light but also extremely strong metal, especially when alloyed with small amounts of aluminum or tin. Manganese is useful in the steel industry, both as an agent for sulfur removal and as a hardening agent in the alloy.

Use of the transition elements is not restricted to metallurgy, however. Cobalt historically was used to add blue tint to glass, and presently is still utilized to remove the tint in clear glass caused by the presence of trace iron impurities. Trace amounts of certain transition elements are also responsible for the characteristic colors of certain gemstones. For example, the red color of a ruby is caused by trace amounts of chromium in the basic crystalline structure of the stone.

Nickel is extremely versatile, even for a transition element. It is used to make hardened steel and also utilized in the formation of nonferrous alloys. Pure nickel is chemically inert and thus used to coat materials in certain manufacturing processes that require the use of highly corrosive materials. An important example of this application was the use of nickel to line the miles of pipe utilized in a gaseous diffusion plant in Oak Ridge, Tennessee, designed to separate uranium, in the form of highly corrosive uranium hexafluoride, into its isotopes for the isolation of material to use in the atomic bomb.

The importance of transition elements in their metallic states should be obvious from the preceding paragraphs. Equally important, however, are the uses of these elements when they exist in chemical compounds rather than as free elements. Transition metal compounds are important in both chemical and biological processes.

In industry, the most important use of transition element compounds is as catalysts. A catalyst is a material that causes a reaction to proceed to completion more rapidly than it would in the absence of the catalyst. The catalyst undergoes no net change in the chemical process.

Often, chemical reactions form more than one product. Certain selective catalysts are useful because they cause a reaction to form only one of the possible products.

By far the most familiar transition metal catalyst is the platinum complex in catalytic converters, used to remove pollutants from automobile exhausts. This is only one of many catalytic applications of platinum. One particular advantage of platinum is that it can easily be isolated in a very fine, powdery form called platinum black. This finely divided platinum has a tremendous surface area, which is of primary importance in catalytic processes. As the surface area of the catalyst in contact with the reaction mixture increases, the length of time required for complete reaction decreases. Platinum in this form is also utilized as an electrode material in certain electrochemical cells, for the same reason. While platinum is probably the most versatile bulk catalyst, other transition elements are utilized, as well. For example, an iron oxide catalyst is used to produce ammonia from nitrogen and hydrogen in the Haber process.

Many coordination complexes are useful in catalysis. They are often more desirable than bulk-metal surfaces for two reasons. First, complexes can dissolve in solution and so are in intimate contact with the reaction mixture, which increases the reaction rate. In addition, complexes tend to be more selective as catalysts leading to higher product yield and purity. Many of the transition elements form complexes of use in industry as catalysts. For example, a ruthenium complex is utilized to form synthesis gas, a term used to describe mixtures of hydrogen and carbon monoxide, which have a variety of applications. A rhodium complex is used in the production of acetic acid, the main ingredient of vinegar, from methanol.

Increasing utilization of coordination complexes in industry has followed much fundamental research focused on understanding the structure and bonding characteristics of complexes. Coordination chemistry has begun to bridge the gap between organic and inorganic chemistry with the isolation of what are known as organometallic complexes. These are materials in which an organic, or carbon-based, ligand is attached to a metal atom or ion. These complexes display reactivities that reflect the unusual nature of the metal-carbon bond.

Biologically important molecules incorporating transition metal ions are examples of naturally occurring organometallic compounds. Copper, cobalt, iron, and molybdenum all are examples of biologically vital transition elements. The function of vitamin B12 depends on the presence of a cobalt-carbon bond. Hemoglobin, which transports oxygen in the human body, contains four heme units, each of which includes an iron atom. These iron atoms can each bind one molecule of oxygen. Iron complexes are also vital to the electron transfer processes in respiration, while copper is found in a variety of proteins in both plants and animals.

Context

Copper, silver, and gold are metals that can be found in their pure, metallic state in nature. This being the case, these are undoubtedly the first known transition elements. Meteorites provided a limited amount of a fourth pure metal, iron. The history of metallurgy is in many ways a history of the transition metals. However, the discovery of new metals and alloys, as well as the refinement of techniques to obtain and purify them, led to many advances without which modern civilization could not exist. Two great epochs in human history, the Bronze and Iron Ages, resulted from utilization of transition elements. Improvements in iron recovery developed at the end of the eighteenth century opened the way to the industrial revolution by providing abundant, inexpensive iron and steel. This resulted in an outburst of new machines and tremendous improvement in construction capabilities, so that complicated bridges and buildings could be erected.

Discovery of additional transition elements continued throughout the nineteenth century. Rhenium, the last naturally occurring transition element to be discovered, was first reported in 1925. One transition element was not isolated until the nuclear age, as it is radioactive with all isotopes having such a short half-life that no hope of finding a natural deposit exists.

Technetium was finally isolated in 1937 by bombarding a molybdenum sample with deuterons, which are heavy hydrogen nuclei.

Coordination complexes of transition elements were studied intensively by Alfred Werner, who would later receive a Nobel Prize for his work. Werner synthesized many complexes and elucidated much information about their structure and bonding. Much of modern inorganic chemistry focuses on coordination complexes and their structure and reactivity.

Understanding bonding in complexes and other properties facilitated development of crystal field and molecular orbital theories to explain the bonding in these compounds.

With the development of complexes containing predominantly organic ligands, a totally new area of chemistry opened up. Organometallic complexes are interesting chemically in terms of the structure and bonding present, and in industry are quite useful because of the special properties of the metal-carbon bond.

With the increased emphasis in the chemical industry on specialty chemicals, undoubtedly transition metal complexes and organometallic compounds will continue to be utilized in novel manufacturing processes and chemical syntheses.

While much has been learned about biologically important molecules containing transition metals, much remains to be learned about their specific functioning in biological systems. This is especially so for those elements that are necessary in the body only in trace amounts.

Principal terms

ATOMIC ORBITAL: the region of space occupied by a particular pair of electrons having a characteristic shape, energy, and orientation

COORDINATION COMPLEX: a chemical compound with a central transition metal ion surrounded by several chemical species via covalent bonds

COVALENT BOND: a chemical bond in which a single pair of electrons is shared between two atoms

ION: an atom possessing a positive or negative electrical charge resulting from loss or gain of electrons, respectively

LIGAND: a neutral molecule or charged ion that donates a pair of electrons to form a metal-ligand bond in a coordination complex

METAL: an element possessing high luster, malleability, and excellent thermal and electrical conductivity

OXIDATION STATE: an indication of the electrical charge possessed by a metal ion in a compound

Bibliography

Asimov, Isaac. ASIMOV'S CHRONOLOGY OF SCIENCE AND DISCOVERY. New York: Harper & Row, 1989. Geared to a general audience, this work places significant advances in science and technology in historical perspective. Includes information about the discovery of most transition elements. Discusses significant technological advances associated with some of the transition elements.

Greenwood, Norman N., and Alan Earnshaw. CHEMISTRY OF THE ELEMENTS. Oxford: Pergamon Press, 1984. This comprehensive book describes every element, including discussions of terrestrial abundance, methods of isolation, properties, uses, and chemical reactivities. The sections on isolation and uses are extremely well written and easy to understand. Also included are special sections detailing elements with particular importance.

Grubbs, Robert H., and William Tumas. "Polymer Synthesis and Organotransition Metal Chemistry." SCIENCE 243 (February 17, 1989): 907. Outlines recent work with transition metal complexes and their application in the production of polymers. A good summary of a rapidly expanding field.

Kostic, Nenad M. "Organometallics: Workhorses of Chemistry." In YEARBOOK OF SCIENCE AND THE FUTURE: 1990, edited by David Calhoun. Chicago: Encyclopaedia Britannica, 1989. Well-illustrated summary of organometallic chemistry, with much information about transition metal complexes and their uses.

Nance, Lewis E. "Electronic Structure Prediction for Transition Metal Ions." THE JOURNAL OF CHEMICAL EDUCATION 61 (April, 1984): 339. Prediction of the electron structure of transition metal ions is difficult because of the overlapping electron shells. This article deals with the prediction of these configurations.

Photon Interactions with Molecules