Manganese is a chemical element; it has the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. First isolated in the 1770s, manganese is a transition metal with many industrial alloy uses, particularly in stainless steels in which it improves strength, workability, and resistance to wear. Various manganese oxides are used as oxidising agents, as rubber additives, and in glass making, fertilizers, and ceramics. Manganese sulfate can be used as a fungicide.
Manganese is also an essential human dietary element, important in macronutrient metabolism, bone formation, and free radical defense systems. It is a critical component in dozens of proteins and enzymes. It is found mostly in the bones, but also the liver, kidneys, and brain. In the human brain, manganese is bound to manganese metalloproteins, most notably glutamine synthetase in astrocytes.
Manganese in the form of the deep violet salt potassium permanganate is commonly used in laboratories as an oxidizer. Potassium permanganate is also used as a biocide in water treatment.
It occurs at the active sites in some enzymes. Electronic-book . Of particular interest is the use of a Mn–O cluster, the oxygen-evolving complex, in the production of oxygen by plants.
Characteristics Physical properties Manganese is a silvery-gray metal that resembles iron. It is hard and very brittle, difficult to melt, but oxidizes easily. Manganese and its common ions are paramagnetic. Manganese tarnishes slowly in air and oxidizes ("rusts") like iron in water containing dissolved oxygen.
Isotopes
Naturally occurring manganese is composed of one stable isotope, Mn. Several radioisotopes have been isolated and described, ranging from Mn to Mn; the most stable ones are with a half-life of 3.7 million years, Mn with a half-life of 312.08 days, and Mn with a half-life of 5.591 days. All of the remaining radioactive isotopes have half-lives of less than three hours, and the majority of less than one minute. The primary decay mode in isotopes lighter than the most abundant stable isotope, Mn, is electron capture, and the primary mode in heavier isotopes is beta decay. Manganese also has three meta states.
Manganese is part of the iron group of elements, which are thought to be synthesized in massive stars shortly before the supernova explosion. Mn decays to Cr with a half-life of 3.7 million years. Because of its short half-life, Mn is relatively rare; it is produced by the impact of cosmic rays on iron.
Chromium and manganese are found together sufficiently for measurement of both to find application in isotope geology, and the Mn/Cr ratios here for dating the early Solar System. Mn–Cr isotopic ratios reinforce the evidence from Al and Pd for the early history of the Solar System. Variations in Cr/Cr and Mn/Cr ratios from several meteorites suggest a non-zero initial Mn/Mn ratio, which indicate that Cr isotopic composition variations must result from in situ decay of Mn in differentiated planetary bodies. Hence, Mn provides additional evidence for nucleosynthetic processes immediately before the coalescence of the Solar System.
Allotropes
Four allotropes (structural forms) of solid manganese are known, labeled α, β, γ and δ, and occur at successively higher temperatures. All are metallic, stable at standard pressure, and have a cubic crystal lattice, but they vary widely in their atomic structures.
Alpha manganese (α-Mn) is the equilibrium phase at room temperature. It has a body-centered cubic lattice and is unusual among elemental metals in that it has a very complex unit cell, with 58 atoms per cell (29 atoms per primitive unit cell) with manganese atoms in four different types of surroundings (sites). It is paramagnetic at room temperature and antiferromagnetic at temperatures below .
Beta manganese (β-Mn) forms when heated above the transition temperature of . It has a primitive cubic structure with 20 atoms per unit cell at two types of sites, which is as complex as that of any other elemental metal. It is easily obtained as a metastable phase at room temperature by rapid quenching of manganese at in ice water. It does not show magnetic ordering, remaining paramagnetic down to the lowest temperature measured (1.1 K).
Gamma manganese (γ-Mn) forms when heated above . It has a simple face-centered cubic structure (four atoms per unit cell). When quenched to room temperature it converts to β-Mn, but it can be stabilized at room temperature by alloying it with at least 5 percent of other elements (such as C, Fe, Ni, Cu, Pd or Au). These solute-stabilized alloys distort into a face-centered tetragonal structure.
Delta manganese (δ-Mn) forms when heated above and is stable up to the manganese melting point of . It has a body-centered cubic structure (two atoms per cubic unit cell).
Chemical compounds
Common oxidation states of manganese are +2, +4, and +7, although all oxidation states from −3 to +7 have been observed. Manganese in oxidation state +7 is represented by salts of the intensely purple permanganate anion . Potassium permanganate is a commonly used laboratory reagent because of its oxidizing properties; it is used as a topical medicine (for example, in the treatment of fish diseases). Solutions of potassium permanganate were among the first stains and fixatives to be used in the preparation of biological cells and tissues for electron microscopy.
Aside from various permanganate salts, Mn(VII) is represented by the unstable, volatile derivative . Oxyhalides ( and ) are powerful oxidizing agents. The most prominent example of Mn in the +6 oxidation state is the green anion manganate, . Manganate salts are intermediates in the extraction of manganese from its ores. Compounds with oxidation states +5 are somewhat elusive, and often found associated to an oxide () or nitride () ligand. One example is the blue anion hypomanganate .
Mn(IV) is somewhat enigmatic because it is common in nature but far rarer in synthetic chemistry. The most common Mn ore, pyrolusite, is . It is the dark brown pigment of many cave drawings and is also a common ingredient in dry cell batteries. Complexes of Mn(IV), such as in , are known but are rarer than those of manganese in the lower oxidation states. Mn(IV)-OH complexes are an intermediate in some enzymes, including the oxygen-evolving center (OEC) in plants.
Simple derivatives of are rarely encountered but can be stabilized by suitably alkaline ligands. Manganese(III) acetate is an oxidant useful in organic synthesis. Solid compounds of manganese(III) are characterized by a strong purple-red color and a preference for distorted octahedral coordination resulting from the Jahn-Teller effect. A particularly common oxidation state for manganese in aqueous solution is +2, which has a pale pink color. Many manganese(II) compounds are known, such as the aquo complexes derived from manganese(II) sulfate () and manganese(II) chloride (). This oxidation state is also seen in the mineral rhodochrosite (manganese(II) carbonate). Manganese(II) commonly exists with a high-spin ground state, with 5 unpaired electrons, because of its high pairing energy. There are no spin-allowed d–d transitions in manganese(II), which explain its faint color.. Oxidation states of manganese −3 −2 −1 0 +1 +2 , , MnO +3 , , +4 +5 +6 +7 , Common oxidation states are in bold.
Organomanganese compounds
Manganese forms a large variety of organometallic derivatives, i.e., compounds with Mn-C bonds. The organometallic derivatives include numerous examples of Mn in its lower oxidation states, i.e. Mn(−III) up through Mn(I). This area of organometallic chemistry is attractive because Mn is inexpensive and of relatively low toxicity.
Of greatest commercial interest is methylcyclopentadienyl manganese tricarbonyl (MMT), which is used as an anti-knock compound added to gasoline in some countries, featuring Mn(I). Consistent with other aspects of Mn(II) chemistry, manganocene () is high-spin. In contrast, its neighboring metal, iron, forms an air-stable, low-spin derivative in the form of ferrocene (). When conducted under an atmosphere of carbon monoxide, reduction of Mn(II) salts gives dimanganese decacarbonyl , an orange and volatile solid. The air-stability of this Mn(0) compound (and its many derivatives) reflects the powerful electron-acceptor properties of carbon monoxide. Many alkene complexes and alkyne complexes are derived from .
In (dmpe = , Mn(II) is low spin, which contrasts with the high spin character of its precursor, . Polyalkyl and polyaryl derivatives of manganese often exist in higher oxidation states, reflecting the electron-releasing properties of alkyl and aryl ligands. One example is .
History The origin of the name manganese is complex. In ancient times, two black minerals were identified from the regions of the Magnetes (either Magnesia, located within modern Greece, or Magnesia ad Sipylum, located within modern Turkey). They were both called magnes from their place of origin, but were considered to differ in sex. The male magnes attracted iron, and was the iron ore now known as lodestone or magnetite, and which probably gave us the term magnet. The female magnes ore did not attract iron, but was used to decolorize glass. This female magnes was later called magnesia, known now in modern times as pyrolusite or manganese dioxide. Neither this mineral nor elemental manganese is magnetic. In the 16th century, manganese dioxide was called manganesum (note the two Ns instead of one) by glassmakers, possibly as a corruption and concatenation of two words, since alchemists and glassmakers eventually had to differentiate a magnesia nigra (the black ore) from magnesia alba (a white ore, also from Magnesia, also useful in glassmaking). Italian physician Michele Mercati called magnesia nigra manganesa, and finally the metal isolated from it became known as manganese (). The name magnesia was eventually used to refer only to the white magnesia alba (magnesium oxide), which provided the name magnesium for the free element when it was isolated much later.
Manganese dioxide, which is abundant in nature, has long been used as a pigment. The cave paintings in Gargas that are 30,000 to 24,000 years old are made from the mineral form of pigments.
Manganese compounds were used by Egyptian and Roman glassmakers, either to add to, or remove, color from glass. Use as "glassmakers soap" continued through the Middle Ages until modern times and is evident in 14th-century glass from Venice.
Because it was used in glassmaking, manganese dioxide was available for experiments by alchemists, the first chemists. Ignatius Gottfried Kaim (1770) and Johann Glauber (17th century) discovered that manganese dioxide could be converted to permanganate, a useful laboratory reagent. By the mid-18th century, the Swedish chemist Carl Wilhelm Scheele used manganese dioxide to produce chlorine. First, hydrochloric acid, or a mixture of dilute sulfuric acid and sodium chloride was made to react with manganese dioxide, and later hydrochloric acid from the Leblanc process was used and the manganese dioxide was recycled by the Weldon process.
Scheele and others were aware that pyrolusite (mineral form of manganese dioxide) contained a new element. Johan Gottlieb Gahn isolated an impure sample of manganese metal in 1774, which he did by reducing the dioxide with carbon. Ignatius Gottfried Kaim also may have reduced manganese dioxide to isolate the metal, but that is uncertain.