MOLYBDENUM: STRUCTURE, PROPERTIES, VALENCES, FUNCTIONS - CHEMISTRY - 2025

The molybdenum (Mo) is a transition metal belonging to group 6, period 5 of the Periodic Table. It has electronic configuration (Kr) 4d ⁵ 5s ¹; atomic number 42 and average atomic mass of 95.94 g / mol. It has 7 stable isotopes: ⁹² Mo, ⁹⁴ Mo, ⁹⁵ Mo, ⁹⁶ Mo, ⁹⁷ Mo, ⁹⁸ Mo and ¹⁰⁰ Mo; being the isotope ⁹⁸ Mo the one that is in greater proportion.

It is a white metal with a silver appearance and has chemical properties similar to chromium. In fact, both are metallic elements of the same group, chromium being located above molybdenum; that is, molybdenum is heavier and has a higher energy level.

Source: By Hi-Res Images of Chemical Elements (http://images-of-elements.com/molybdenum.php), via Wikimedia Commons

Molybdenum is not found free in nature, but as part of minerals, the most abundant being molybdenite (MoS ₂). In addition, it is associated with other sulfurous minerals, from which copper is also obtained.

Its use increased during the First World War, as it replaced tungsten, which was scarce due to its massive exploitation.

characteristics

Molybdenum is characterized by its great durability, resistance to corrosion, high melting point, being malleable and withstanding high temperatures. It is considered a refractory metal because it has a melting point higher than platinum (1,772º C).

It also has a set of additional properties: the binding energy of its atoms is high, low vapor pressure, low coefficient of thermal expansion, high level of thermal conductance and low electrical resistance.

All these properties and characteristics have allowed molybdenum to have numerous uses and applications, the most notable of which is the formation of alloys with steel.

On the other hand, it is an essential trace element for life. In bacteria and plants, molybdenum is a cofactor present in numerous enzymes involved in the fixation and utilization of nitrogen.

Molybdenum is a cofactor for the activity of oxotransferase enzymes, which transfer oxygen atoms from water while transferring two electrons. These enzymes include primate xanthine oxidase, whose function is to oxidize xanthine to uric acid.

It can be obtained from several foods, including the following: cauliflower, spinach, garlic, whole grains, buckwheat, wheat germ, lentils, sunflower seeds, and milk.

Discovery

Molybdenum is not isolated in nature, so in many of its complexes it was confused in ancient times with lead or carbon.

In 1778, Carl Wilhelm, a Swedish chemist and pharmacist, managed to identify molybdenum as a distinct element. Wilhelm treated molybdenite (MoS ₂) with nitric acid, obtaining a compound of an acid nature in which he identified molybdenum.

Later, in 1782, Peter Jacob Hjelm, using Wilhelm's acid compound, by reduction with carbon, succeeded in isolating an impure molybdenum.

Structure

What is the crystal structure of molybdenum? Its metal atoms adopt the body-centered cubic crystal system (bcc) at atmospheric pressure. At higher pressures, molybdenum atoms compact to form denser structures, such as face-centered cubic (fcc) and hexagonal (hcp).

Its metallic bond is strong, and coincides with the fact that it is one of the solids with the highest melting point (2623ºC). This structural strength is due to the fact that molybdenum is rich in electrons, its crystalline structure is considerably dense, and it is heavier than chromium. These three factors allow it to strengthen the alloys in which it is a part.

On the other hand, more important than the structure of metallic molybdenum is that of its compounds. Molybdenum is characterized by its ability to form dinuclear (Mo-Mo) or polynuclear (Mo-Mo-Mo- ···) compounds.

It can also coordinate with other molecules to form compounds with formulas MoX ₄ through MoX ₈. Within these compounds, the presence of oxygen bridges (Mo-O-Mo) or sulfur (Mo-S-Mo) is common.

Properties

Appearance

Solid silvery white.

Melting point

2,623 ° C (2,896 K).

Boiling point

4,639 ° C (4,912 K).

Enthalpy of fusion

32 kJ / mol.

Enthalpy of vaporization

598 kJ / mol.

Vapor pressure

3.47 Pa at 3,000 K.

Hardness on the Mohs scale

5.5

Water solubility

Molybdenum compounds are poorly soluble in water. However, the molybdate ion MoO ₄ ^-2 is soluble.

Corrosion

It is resistant to corrosion and is one of the metals that best resists the action of hydrochloric acid.

Oxidation

It does not rust at room temperature. To rust quickly it requires temperatures higher than 600 ºC.

Valencias

The electron configuration of molybdenum is 4d ⁵ 5s ¹, so it has six valence electrons. Depending on which atom it binds to, the metal can lose all its electrons and have a valence of +6 (VI). For example, if it forms bonds with the electronegative fluorine atom (MoF ₆).

However, it can lose 1 to 5 electrons. Thus, its valences span the range from +1 (I) to +5 (V). When you lose only one electron, it leaves the 5s orbital, and its configuration is 4d ⁵. The five electrons of the 4d orbital require highly acidic media and electron-like species to leave the Mo atom.

Of its six valences, which are the most common? The +4 (IV) and +6 (VI). Mo (IV) has a 4d ² configuration, while Mo (VI),.

For Mo ⁴⁺ it is not clear why it is more stable than, for example, Mo ³⁺ (as is the case with Cr ³⁺). But for Mo ^{6+ it} is possible to lose these six electrons because the noble gas krypton becomes isoelectronic.

Molybdenum chlorides

A series of molybdenum chlorides with different valences or oxidation states are listed below, from (II) to (VI):

-Molybdenum dichloride (MoCl ₂). Yellow solid.

-Molybdenum trichloride (MoCl ₃). Dark red solid.

-Molybdenum tetrachloride (MoCl ₄). Solid black.

-Molybdenum pentachloride (MoCl ₅). Solid dark green.

-Molybdenum hexachloride (MoCl ₆). Solid brown.

Functions in the body

Molybdenum is an essential trace element for life, as it is present as a cofactor in numerous enzymes. Oxotransferases use molybdenum as a cofactor to fulfill their function of transferring oxygen from water with a pair of electrons.

Among the oxotransferases are:

• Xanthine oxidase.
• Aldehyde oxidase, which oxidizes aldehydes.
• Amines and sulfides in the liver.
• Sulfite oxidase, which oxidizes sulfite in the liver.
• Nitrate reductase.
• Nitrite reductase present in plants.

Xanthine enzyme

The enzyme xanthine oxidase catalyzes the terminal step in the catabolism of purines in primates: the conversion of xanthine to uric acid, a compound that is then excreted.

Xanthine oxidase has FAD as a coenzyme. In addition, non-heme iron and molybdenum are involved in the catalytic action. The action of the enzyme can be described by the following chemical equation:

Xanthine + H ₂ O + O ₂ => Uric Acid + H ₂ O ₂

Molybdenum acts as the cofactor molybdopterin (Mo-co). Xanthine oxidase is found primarily in the liver and small intestine, but the use of immunological techniques has allowed its location in the mammary glands, skeletal muscle, and the kidney.

The enzyme xanthine oxidase is inhibited by the drug Allopurinol, used in the treatment of gout. In 2008, the commercialization of the drug Febuxostat began with a better performance in the treatment of the disease.

Aldehyde oxidase enzyme

The enzyme aldehyde oxidase is located in the cell cytoplasm, being found both in the vegetable kingdom and in the animal kingdom. The enzyme catalyzes the oxidation of the aldehyde to carboxylic acid.

Cytochrome P ₄₅₀ and the intermediates of the enzyme monoamine oxidase (MAO) also catalyze oxidation.

Due to its broad specificity, the enzyme aldehyde oxidase can oxidize many drugs, performing its function mainly in the liver. The action of the enzyme on the aldehyde can be outlined in the following way:

Aldehyde + H ₂ O + O ₂ => Carboxylic Acid + H ₂ O ₂

Sulfite oxidase enzyme

The enzyme sulfite oxidase is involved in the conversion of sulfite to sulfate. This is the terminal step in the degradation of sulfur-containing compounds. The reaction catalyzed by the enzyme occurs according to the following scheme:

SO ₃ ^-2 + H ₂ O + 2 (Cytochrome C) oxidized => SO ₄ ^-2 + 2 (Cytochrome C) reduced + 2 H ⁺

A deficiency of the enzyme due to a genetic mutation in man can lead to premature death.

Sulfite is a neurotoxic compound, so a low activity of the enzyme sulfite oxidase can lead to mental illness, mental retardation, mental degradation and ultimately death.

In iron metabolism and as a component of teeth

Molybdenum is involved in the metabolism of iron, facilitating its intestinal absorption and the formation of erythrocytes. In addition, it is part of the enamel of the teeth, and together with fluoride it helps in the prevention of cavities.

Deficiency

A deficiency in molybdenum intake has been linked to a higher incidence of esophageal cancer in regions of China and Iran, compared to regions of the United States with high levels of molybdenum.

Importance in Plants

Nitrate reductase is an enzyme that plays a vital role in plants, since together with the enzyme nitrite reductase it intervenes in the transformation of nitrate into ammonium.

The two enzymes require the cofactor (Mo-co) for their operation. The reaction catalyzed by the enzyme nitrate reductase can be outlined as follows:

Nitrate + Electron Donor + H ₂ O => Nitrite + Oxidized Electron Donor

The nitrate oxidation-reduction process occurs in the cytoplasm of plant cells. Nitrite, product of the previous reaction, is transferred to the plastid. The enzyme nitrite reductase acts on nitrite, causing ammonia.

Ammonium is used to synthesize amino acids. In addition, plants use molybdenum in the conversion of inorganic phosphorus to organic phosphorus.

Organic phosphorus exists in numerous biological function molecules, such as: ATP, glucose-6-phosphate, nucleic acids, pholipids, etc.

A molybdenum deficiency mainly affects the group of crucifers, legumes, poinsettias, and primroses.

In cauliflower, a molybdenum deficiency causes a restriction of the width of the leaf blade, a reduction in plant growth and flower formation.

Uses and applications

Catalyst

-It is a catalyst for the desulfurization of petroleum, petrochemicals and liquids derived from coal. The catalyst complex comprises MoS ₂ fixed on alumina, and activated by cobalt and nickel.

-Molybdate forms a complex with bismuth for the selective oxidation of propene, ammonia and air. Thus, they form acrylonitrile, acetonitrile and other chemicals, which are raw materials for the plastics and fiber industries.

Similarly, iron molybdate catalyzes the selective oxidation of methanol to formaldehyde.

Pigments

-Molybdenum is involved in the formation of pigments. For example, molybdenum orange is formed by the co-precipitation of lead chromate, lead molybdate, and lead sulfate.

This is a light pigment that is stable at various temperatures, appearing in bright red, orange or red-yellow colors. It is used in the preparation of paints and plastics, as well as in rubber and ceramic products.

Molybdate

-Molybdate is a corrosion inhibitor. Sodium molybdate has been used as a replacement for chromate to inhibit corrosion of tempered steels at a wide pH range.

-It is used in water coolers, air conditioners and heating systems. Molybdates are also used to inhibit corrosion in hydraulic systems and in automotive engineering. Also, pigments that inhibit corrosion are used in paints.

-Molybdate, due to its properties of high melting point, low coefficient of thermal expansion and high thermal conductivity, is used to produce tapes and threads used by the lighting industry.

-Used in motherboards for semiconductors; in power electronics; electrodes for glass melting; chambers for high temperature furnaces and cathodes for coating solar cells and flat screens.

-And also, molybdate is used in the production of crucibles for all the usual processes in the field of sapphire processing.

Alloys with steel

-Molybdenum is used in alloys with steel that withstand high temperatures and pressures. These alloys are used in the construction industry and in the manufacture of parts for aircraft and automobiles.

-Molybdate, even at concentrations as low as 2%, gives its alloy with steel a high resistance to corrosion.

Other uses

-Molybdate is used in the aerospace industry; in the manufacture of LCD screens; in water treatment and even in the application of the laser beam.

-Molybdate disulfide is, by itself, a good lubricant and provides extreme pressure tolerance properties in the interaction of lubricants with metals.

Lubricants form a crystalline layer on the surface of metals. Thanks to this, metal-metal friction is reduced to a minimum, even at high temperatures.

References

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