Chromium molybdenum. Metals of group VI of the secondary subgroup (Cr, Mo, W)

The sixth group of elements of the periodic table includes chromium 24 Cr, molybdenum 42 Mo, tungsten 74 W and the radioactive metal seaborgium 106 Sg. Chromium occurs in nature in the form of four stable isotopes, of which 52 Cr predominates (83.8%). Natural molybdenum and tungsten are a complex mixture of seven and five isotopes, respectively, most of which occur in comparable quantities in the earth's crust. Thus, the dominant nuclide molybdenum-98 makes up only 24% of the total number of molybdenum atoms.

In 1778, the Swedish chemist K. Scheele obtained the oxide MoO 3 from the mineral molybdenite MoS 2, during the reduction of which with coal four years later R. Hjelm isolated a new element - molybdenum. Its name comes from the Greek “molybdos” - lead. The confusion stems from the fact that soft materials such as graphite, lead and molybdenite MoS 2 were previously used as writing leads. This is associated with the name of graphite “black lead” - black lead.

In 1781, K. Scheele and T. Wergmann isolated the oxide of a new element from the mineral CaWO 4 (scheelite). Two years later, Spanish chemists - brothers J. and F. d'Eloire showed that the same element is integral part mineral (Fe, Mn)WO 4 – wolframite. Its name comes from the German Wolf Rahm - wolf foam. When smelting tin, a large amount of metal was lost, turning into slag. This was caused by the fact that wolframite, accompanying cassiterite, interfered with the reduction of tin. Medieval metallurgists said that wolframite devours tin like a wolf eats a sheep. By reducing wolframite with coal, they obtained a new metal called tungsten.

In 1797, the French chemist L. Vauquelin studied the properties of the orange-red mineral crocoite PbCrO 4, sent to him from Siberia by the Russian geologist M. Pallas. When the mineral was boiled with potash, it produced an orange-red solution

3PbCrO 4 +3K 2 CO 3 + H 2 O = Pb 3 (CO 3) 2 (OH) 2 ¯ + 3K 2 CrO 4, + CO 2,

from which he isolated potassium chromate, then chromic anhydride and, finally, by reducing CrO 3 with coal - the new metal chromium. The name of this element comes from the Greek “chroma” - color and is associated with the variety of colors of its compounds. The mineral chromite, the most important modern raw material for chromium production, was found in the Urals in 1798.

Seaborgium was first obtained in 1974 by American scientists under the leadership of Albert Ghiorso in Berkeley (USA). The synthesis of an element in the amount of several atoms was carried out according to the reactions:

18 O + 249 Cf 263 106 Sg + 4 1 n,

248 Cf + 22 Ne 266 106 Sg + 4 1 n

The half-life of the longest-lived isotope 266 Sg is 27.3 s. The element is named after the American physicist and chemist Glenn Seaborg.

Following the general tendencies of filling the d-sublevel when moving through the period for elements of the sixth group, it would be necessary to assume the configuration of the valence electrons in the ground state (n-1)d 4 ns 2, which, however, is realized only in the case of tungsten. In chromium and molybdenum atoms, the energy gain caused by the stabilization of a half-filled sublevel and the complete absence of the destabilizing contribution of pairing energy turns out to be higher than the energy that must be spent on the transition of one of the s-electrons to the d-sublevel. This leads to a “jump” of the electron (see section 1.1) and the electron configuration (n-1)d 5 ns 1 for chromium and molybdenum atoms. The radii of atoms and ions (Table 5.1) increase during the transition from chromium to molybdenum and practically do not change upon further transition to tungsten; their close values ​​for molybdenum and tungsten are a consequence of lanthanide compression. At the same time, despite this, the difference in properties between these two elements turns out to be much more noticeable than between the 4d and 5d elements of the fourth and fifth groups (zirconium and hafnium, niobium and tantalum): as you move away from the third group of influence lanthanide compression on the properties of atoms weakens. The values ​​of the first ionization energies during the transition from chromium to tungsten increase, as for elements of the 5th group.

Table 5.1. Some properties of elements of group 6

* For coordination number CN = 12.

** For coordination number CN = 6.

*** The radius is indicated for low- (ns) and high-spin (hs) states.

**** Unstable oxidation states are indicated in parentheses.

In various compounds, the elements chromium, molybdenum and tungsten exhibit oxidation states from –4 to +6 (Table 5.1). As in other groups of transition metals, the stability of compounds with the highest oxidation state, as well as coordination numbers, increase from chromium to tungsten. Chromium, like other d-metals, in lower oxidation states has a coordination number of 6, for example, 3+, –. As the degree of oxidation increases, the ionic radius of the metal inevitably decreases, which leads to a decrease in its coordination number. That is why, in higher oxidation states in oxygen compounds, chromium has a tetrahedral environment, realized, for example, in chromates and dichromates, regardless of the acidity of the medium. The process of polycondensation of chromate ions, successively leading to dichromates, trichromates, tetrachromates and, finally, to hydrated chromic anhydride, is only a sequential increase in the chain of CrO 4 tetrahedra connected by common vertices. For molybdenum and tungsten, tetrahedral anions, on the contrary, are stable only in an alkaline medium, and upon acidification they increase the coordination number to six. The resulting metal-oxygen octahedra MO 6 condense through common edges into complex isopolyanions that have no analogues in chromium chemistry. As the degree of oxidation increases, the acidic and oxidizing properties increase. Thus, Cr(OH)2 hydroxide exhibits only basic properties, Cr(OH)3 exhibits amphoteric properties, and H2CrO4 exhibits acidic properties.

Chromium(II) compounds are strong reducing agents that are instantly oxidized by atmospheric oxygen (Fig. 5.1. Frost diagram for chromium, molybdenum and tungsten). Their reducing activity (E o (Cr 3+ /Cr 2+) = –0.41 V) is comparable to similar vanadium compounds.

Table 5.2. Stereochemistry of some Cr, Mo and W compounds

The most characteristic oxidation state for chromium is +3 (Fig. 5.1). The high stability of Cr(III) compounds is associated with both thermodynamic factors - the symmetric d 3 configuration, which provides high strength of the Cr(III) - ligand bond due to the high energy of stabilization by the crystal field (ESF) in the octahedral field () of the ligands, and with the kinetic inertness of octahedral chromium(III) cations. Unlike molybdenum and tungsten compounds in higher oxidation states, chromium(VI) compounds – strong oxidizing agents E 0 ( /Cr 3+) = 1.33 V. Chromate ions can be reduced by hydrogen at the time of separation in hydrochloric acid solution to Cr 2+ ions, molybdates to molybdenum(III) compounds, and tungstates to tungsten(V) compounds.

Compounds of molybdenum and tungsten in lower oxidation states contain metal-metal bonds, that is, they are clusters. The best known are octahedral clusters. For example, molybdenum dichloride contains Mo 6 Cl 8: Cl 4 groups. The ligands that make up the cluster ion are bound much more tightly than the external ones, therefore, when exposed to an alcohol solution of silver nitrate, it is possible to precipitate only one third of all chlorine atoms. Metal-metal bonds are also found in some chromium(II) compounds, such as carboxylates.

Despite the close stoichiometry of the compounds of the elements of the sixth group of chromium and the sulfur group, the atoms of which contain the same number of valence electrons, only a distant similarity is observed between them. For example, the sulfate ion has the same dimensions as the chromate and can isomorphically replace it in some salts. Chromium(VI) oxochloride is similar in its ability to hydrolyze to sulfuryl chloride. At the same time, sulfate ions in aqueous solutions practically do not show oxidative properties, and selenates and tellurates do not have the ability to form isopolycompounds, although individual atoms of these elements may be included in their composition.

Compared to d-elements of the fourth and fifth groups, chromium, molybdenum and tungsten cations are characterized by a much higher Pearson “softness”, which increases down the group. The consequence of this is the rich chemistry of sulfide compounds, especially developed in molybdenum and tungsten. Even chromium, which has the greatest rigidity compared to other elements of the group, is capable of replacing the oxygen environment with sulfur atoms: for example, by fusing chromium(III) oxide with potassium thiocyanate, KCrS 2 sulfide can be obtained.

5.2. Prevalence in nature. Preparation and use of simple substances.

The elements of the sixth group are even and therefore more common than the odd elements of the 5th and 7th groups. Their natural galaxy consists of a large number of isotopes (Table 5.1). Chromium is the most common in nature. Its content in the earth's crust is 0.012% wt and is comparable to the abundance of vanadium (0.014% wt) and chlorine (0.013% wt). Molybdenum (3×10 -4% mass) and tungsten (1×10 -4% mass) are rare and trace metals. The most important industrial chromium mineral is chromium iron ore FeCr 2 O 4 . Other minerals are less common - crocoite PbCrO 4, chrome ocher Cr 2 O 3. The main form of occurrence of molybdenum and tungsten in nature is feldspars and pyroxenes. From molybdenum minerals highest value has molybdenite MoS 2, mainly due to the fact that it does not contain significant quantities of other metals, which greatly facilitates the processing of ore. The products of its oxidation into natural conditions are wulfenite PbMoO 4 and powellite CaMoO 4 . The most important tungsten minerals are scheelite CaWO 4 and wolframite (Fe,Mn)WO 4 , but the average tungsten content in the ores is extremely low - no more than 0.5%. Due to the similar properties of molybdenum and tungsten, complete solid solutions of CaMoO4-CaWO4 and PbMoO4-PbWO4 exist.

For many technical purposes, there is no need to separate the iron and chromium contained in chromium iron ore. An alloy formed when it is reduced with coal in electric furnaces

FeCr 2 O 4 + 4C Fe + 2Cr + 4CO,

Ferrochrome is widely used in the production of stainless steels. If silicon is used as a reducing agent, ferrochrome with a low carbon content is obtained, which is used for the production of strong chrome steels.

Pure chromium is synthesized by reduction of Cr 2 O 3 oxide with aluminum

Сr 2 O 3 + 2Al = 2Cr + Al 2 O 3

or silicon

2Cr 2 O 3 + 3Si = 4Cr + 3SiO 2.

In the aluminothermic method, a preheated mixture of chromium(III) oxide and aluminum powder with oxidizing agent additives (Footnote: the heat released during the reduction of chromium oxide with aluminum is not enough for the process to occur spontaneously. Potassium dichromate, barium peroxide, chromic anhydride are used as an oxidizing agent) is loaded into the crucible. The reaction is initiated by igniting a mixture of aluminum and sodium peroxide. The purity of the resulting metal is determined by the content of impurities in the original chromium oxide, as well as in reducing agents. It is usually possible to obtain metal of 97-99% purity, containing small amounts of silicon, aluminum and iron.

To obtain the oxide, chromium iron ore is subjected to oxidative melting in an alkaline environment

4FeCr 2 O 4 + 8Na 2 CO 3 + 7O 2 8Na 2 CrO 4 + 2Fe 2 O 3 + 8CO 2,

and the resulting Na 2 CrO 4 chromate is treated with sulfuric acid.

2Na 2 CrO 4 + 2H 2 SO 4 = Na 2 Cr 2 O 7 + 2NaHSO 4 + H 2 O

In some industrial plants, carbon dioxide is used instead of sulfuric acid, carrying out the process in autoclaves under a pressure of 7 - 15 atm.

2Na 2 CrO 4 + H 2 O + 2CO 2 = Na 2 Cr 2 O 7 + 2NaHCO 3.

At normal pressure the equilibrium of the reaction is shifted to the left.

Then the crystallized sodium bichromate Na 2 Cr 2 O 7 × 2H 2 O is dehydrated and reduced with sulfur or coal

Na 2 Cr 2 O 7 + 2C Cr 2 O 3 + Na 2 CO 3 + CO.

The purest chromium in industry is obtained either by electrolysis of a concentrated aqueous solution of chromic anhydride in sulfuric acid, a solution of chromium(III) sulfate Cr 2 (SO 4) 3 or chromium-ammonium alum. Chromium with a purity greater than 99% is released on a cathode made of aluminum or stainless steel. Complete cleaning The removal of nitrogen or oxygen impurities from the metal is achieved by keeping the metal in a hydrogen atmosphere at 1500 °C or by distillation in a high vacuum. The electrolytic method makes it possible to obtain thin films of chromium, which is why it is used in electroplating.

To obtain molybdenum, ore enriched by flotation is roasted

900 – 1000 ºС

2MoS 2 + 7O 2 = 2MoO 3 + 4SO 2.

The resulting oxide is distilled off at the reaction temperature. Then it is further purified by sublimation or dissolved in an aqueous solution of ammonia

3MoO 3 + 6NH 3 + 3H 2 O = (NH 4) 6 Mo 7 O 24,

recrystallize and decompose again in air to the oxide. Metal powder is obtained by reducing the oxide with hydrogen:

MoO 3 + 3H 2 = Mo + 3H 2 O,

pressed and melted in an arc furnace in an atmosphere of inert gas or converted into an ingot using powder metallurgy. Its essence lies in the production of products from fine powders by cold pressing molding and subsequent high-temperature treatment. The technological process of manufacturing products from metal powders includes preparing the mixture, molding blanks or products, and sintering them. Molding is carried out by cold pressing under high pressure (30–1000 MPa) in metal molds. Sintering of products from homogeneous metal powders is carried out at temperatures reaching 70–90% of the melting temperature of the metal. To avoid oxidation, sintering is carried out in an inert, reducing atmosphere or in a vacuum. Thus, molybdenum powder is first pressed in steel molds . After preliminary sintering (at 1000-1200 °C) in a hydrogen atmosphere, the workpieces (stubs) are heated to 2200-2400 °C. In this case, individual crystallites melt from the surface and stick together, forming a single ingot, which is subjected to forging.

The starting material for the production of tungsten is its oxide WO 3 . To obtain it, ore (scheelite CaWO 4 or wolframite FeWO 4), previously enriched by flotation in solutions of surfactants, is subjected to alkaline or acid opening. Alkaline dissection is carried out by decomposing the concentrate in autoclaves with a soda solution at 200 oC

CaWO 4 + Na 2 CO 3 = Na 2 WO 4 + CaCO 3 ¯ .

The equilibrium shifts to the right due to the use of a threefold excess of soda and the precipitation of calcium carbonate. According to another method, wolframite concentrates are decomposed by heating with a strong solution of caustic soda or sintering with soda at 800-900 °C

CaWO 4 + Na 2 CO 3 = Na 2 WO 4 + CO 2 + CaO.

In all cases, the final decomposition product is sodium tungstate, which is leached with water. The resulting solution is acidified and tungstic acid is precipitated

Na 2 WO 4 + 2HCl = H 2 WO 4 ¯ + 2NaCl.

Acidic dissection of scheelite also produces tungstic acid:

CaWO 4 + 2HCl = H 2 WO 4 ¯ + CaCl 2.

The released tungstic acid precipitate is dehydrated

H 2 WO 4 = WO 3 + H 2 O.

The resulting oxide is reduced with hydrogen

WO 3 + 3H 2 = W + 3H 2 O.

The oxide used for the production of high-purity tungsten is pre-purified by dissolution in ammonia, crystallization of ammonium paratungstate and its subsequent decomposition.

When reducing the oxide, tungsten metal is also obtained in the form of a powder, which is pressed and sintered at 1400 ºС, and then the rod is heated to 3000 ºС, passing an electric current through it in a hydrogen atmosphere. Tungsten rods prepared in this way acquire plasticity; from them, for example, tungsten filaments are drawn for incandescent electric lamps. Large-crystalline tungsten and molybdenum ingots are produced by electron beam melting in a vacuum at 3000-3500 o C.

Chromium is used in metallurgy in the production of stainless steels, which have unique corrosion resistance. Adding just a few percent chromium to iron makes the metal more susceptible to heat treatment. Chrome is used to alloy steels used to make springs, springs, tools, and bearings. A further increase in the chromium content in steel leads to a sharp change in its mechanical characteristics– decreased wear resistance, appearance of fragility. This is due to the fact that when the chromium content in steel is more than 10%, all the carbon contained in it goes into the form of carbides. At the same time, such steel is practically not subject to corrosion. The most common grade of stainless steel contains 18% chromium and 8% nickel. The carbon content in it is very low - up to 0.1%. Stainless steel is used to make turbine blades, submarine hulls, as well as pipes, metal tiles, and cutlery. A significant amount of chromium is used for decorative corrosion-resistant coatings, which not only give products a beautiful appearance and increase their service life, but also increase the wear resistance of machine parts and tools. The chrome coating with an underlayer of copper and nickel protects the steel well from corrosion, giving the products a beautiful appearance. Parts of cars, bicycles, and devices are subjected to protective and decorative chrome plating; the thickness of the applied film usually does not exceed 5 microns. In terms of reflectivity, chrome coatings are second only to silver and aluminum, which is why they are widely used in the production of mirrors and spotlights. Nickel alloys containing up to 20% chromium (nichrome) are used for the manufacture of heating elements - they have high resistance and become very hot when current passes. The addition of molybdenum and cobalt to such alloys greatly increases their heat resistance; gas turbine blades are made from such alloys. Along with nickel and molybdenum, chromium is part of metal ceramics, a material used in dental prosthetics. Chromium compounds are used as green (Cr 2 O 3, CrOOH), yellow (PbCrO 4, CdCrO 4) and orange pigments. Many chromates and dichromates are used as corrosion inhibitors (CaCr 2 O 7, Li 2 CrO 4, MgCrO 4), wood preservatives (CuCr 2 O 7), fungicides (Cu 4 CrO 7 ×xH 2 O), catalysts (NiCrO 4, ZnCr 2 O 4). World chromium production currently exceeds 700 thousand tons per year.

Molybdenum is also used in metallurgy to create hard and wear-resistant, chemically resistant and heat-resistant structural alloys, as an alloying additive to armor steels. The thermal expansion coefficients of molybdenum and some types of glass (they are called “molybdenum glass”) are close, therefore, inputs to glass electric vacuum devices and bulbs of powerful light sources are made from molybdenum. Due to its relatively small thermal neutron capture cross section (2.6 barn), molybdenum is used as a structural material in nuclear reactors . Molybdenum wire, tapes and rods serve as heating elements and heat shields in vacuum installations. Molybdenum, alloyed with titanium, zirconium, niobium, and tungsten, is used in aviation and rocketry for the manufacture of gas turbines and engine parts.

Tungsten is the best material for filaments and spirals in incandescent lamps, radio tube cathodes and X-ray tubes. The high operating temperature (2200-2500 o C) provides greater light output, and the low evaporation rate and the ability to hold shape (do not sag when heated to 2900 o C) ensure a long service life of the filaments. Tungsten is also used to create hard, wear-resistant and heat-resistant alloys in mechanical engineering and rocketry. Steels containing 20% ​​tungsten have the ability to self-harden - blades of cutting tools are made from them. Tungsten alloys advantageously combine heat resistance and heat resistance not only in humid air, but also in many aggressive environments. For example, when 10% tungsten is added to nickel, its corrosion resistance increases 12 times. Tungsten-rhenium thermocouples allow measuring temperatures up to 3000 °C.

Chromium, nickel And molybdenum are the most important alloying elements steels. They are used in various combinations and different categories of alloy steels are obtained: chromium, chromium-nickel, chromium-nickel-molybdenum and similar alloy steels.

The influence of chromium on the properties of steels

The tendency of chromium to form carbides is average among otherscarbide-forming alloying elements. At a low Cr/C ratio of chromium content relative to iron, only cementite of the (Fe,Cr) type is formed. 3 C. With an increase in the ratio of chromium to carbon content in Cr/C steel, chromium carbides of the form (Cr,Fe) appear 7 C 3 or (Cr,Fe) 2 3C 6 or both. Chromium increases the ability of steels to be thermally hardened, their resistance to corrosion and oxidation, provides increased strength at elevated temperatures, and also increases the abrasive wear resistance of high-carbon steels.

Chromium carbides are also wear-resistant. They are the ones who provide durability to steel blades - it’s not for nothing that knife blades are made from chromium steels. Complex chromium-iron carbides enter the solid solution of austenite very slowly - therefore, when heating such steels for hardening, a longer exposure at the heating temperature is required. Chromium is rightfully considered the most important alloying element in steels. The addition of chromium to steels causes impurities such as phosphorus, tin, antimony and arsenic to segregate to the grain boundaries, which can cause temper brittleness in steels.

The influence of nickel on the properties of steels

Nickel does not form carbides in steels. In steels it is an element that contributes to the formation and preservation austenite . Nickel increases the hardening of steels. In combination with chromium and molybdenum, nickel further increases the thermal hardening ability of steels and helps to increase the toughness and fatigue strength of steels. Dissolving into ferrite Nickel increases its viscosity. Nickel increases the corrosion resistance of chromium-nickel austenitic steels in non-oxidizing acid solutions.

The influence of molybdenum on the properties of steels

Molybdenum readily forms carbides in steels. It dissolves only slightly in cementite. Molybdenum forms molybdenum carbides once the carbon content of the steel becomes high enough. Molybdenum is capable of providing additional thermal hardening during tempering of hardened steels. It increases the creep resistance of low-alloy steels at high temperatures Oh.

Molybdenum additives help refine the grain of steels and increase the hardening of steels heat treatment, increase the fatigue strength of steels. Alloy steels containing 0.20-0.40% molybdenum or the same amount of vanadium slow down the occurrence of temper brittleness, but do not completely eliminate it. Molybdenum improves the corrosion resistance of steels and is therefore widely used in high-alloy ferritic stainless steels and in chromium-nickel austenitic stainless steels. High molybdenum content reduces the susceptibility of stainless steel to pitting corrosion. Molybdenum has a very strong solid solution strengthening effect on austenitic steels that are used at elevated temperatures.

Any molybdenum alloys are considered heavy, given the presence of a refractory metal as a base. Pure molybdenum with additives or a compound alloyed with other metals has high strength characteristics and is resistant to external factors environment, corrosion, exposure to extremely high temperatures.

Chemical properties and characteristics

Molybdenum occupies a special place among metals. With its help, it is possible to obtain alloys that are used in precision measuring instruments, counterweights, jet engines, screens of melting furnaces, in a wide variety of mechanisms and critical installations.

Mo is located in the 5th group and 5th period in the table chemical elements Mendeleev. The density at normal room temperature is 10,200 kg/m3, and the melting point reaches 2620±10°C. He transmits to alloys amazing properties: heat resistance, strength, reliability, low coefficient. expansion when exposed to high temperatures, insignificant neuronal capture cross section. At the same time, in terms of thermal conductivity, it is inferior to copper, but ahead of iron. In terms of processing, it is simpler compared to tungsten. But the latter refractory metal exhibits better mechanical strength.

In terms of their properties and characteristics, molybdenum alloys are as close as possible to pure metal, especially if the base occupies a large percentage of the total mass. Tungsten-molybdenum alloys are endowed with the best properties of both elements. By varying the ratios of refractory metals in one compound, it is possible to obtain a semi-finished product or a finished product with the required parameters.

Technologists point out that one of the significant disadvantages of Mo is its susceptibility to oxidation at temperatures above 500°C. At the same time, although alloying does not completely solve this problem, it helps to increase heat resistance and reduce fragility (for example, by introducing lanthanum oxide), and increase the time the part is exposed to increased load. When certain components are added, the recrystallization time increases.

Types and features of alloys

Tungsten-molybdenum. From compounds based on refractory metals, crucibles and extruded workpieces, hot-rolled sheets, plates, rings, parts for equipping high-temperature and hydrogen furnaces, and sputtering targets are obtained. With certain processing, it is possible to obtain products of complex shapes.
Nickel-molybdenum alloys. The most common combination, available in various brands. Applicable for alloying steels, they are common in the manufacture of containers/vessels for radioactive elements, having a greater absorption coefficient of gamma rays than lead. Alloying in this case is more economical when compared to using pure Mo. At the same time, the characteristics of the finished products are almost identical. Collimators, dosimetric equipment and protective blocks/screens are also made from such alloys.
Chromium-molybdenum compounds. Chromium increases the strength of the joint, making it heat-resistant and acid-resistant. Alloys with the addition of cobalt are used in the production of artificial teeth, crowns, and bridges. Solid, but at the same time moderately elastic compounds do not corrode and do not react with biological fluids, food and drinks.

In addition to purchasing molybdenum alloys with nickel, tungsten and other metals, it is possible to order additional services - processing semi-finished products and finished parts with various mechanical and by chemical means to give them certain qualities.

How to buy molybdenum alloy profitably?

The company can order the production of heavy alloys based on refractory metals. You can buy molybdenum alloy of both common and rare brands. Before ordering, we recommend that you contact the company’s specialists. Many years of experience as a technologist and a well-functioning production line make it possible to strictly comply with GOST regulations in the production of powder, blanks in the form of ingots and bars, as well as any complex products made of chromium-molybdenum alloys, compounds containing nickel, tungsten, etc. metals in the composition. Call right now and find out about the possibility of placing an order for a batch of the required volume or for the production of parts according to individual drawings.

Program

Chemical activity of metals from the chromium subgroup. Basic valence states. Chromium complex compounds, structure and significance. Hydrate isomerism. Acid-base and redox properties of chromium (II), (III) and (VI) compounds. Polyconnections. Chromium peroxo compounds. Analytical reactions of elements of the chromium subgroup. Comparison of stability, acid-base and redox properties of higher oxygen compounds of elements of the chromium subgroup.

The chromium subgroup is formed by the metals of the secondary subgroup of the sixth group - chromium, molybdenum and tungsten. The outer electronic layer of atoms of elements of the chromium subgroup contains one or two electrons, which determines the metallic nature of these elements and their difference from the elements of the main subgroup. In binary compounds Cr, Mo and W, all oxidation states from 0 to +6 are exhibited, since, in addition to the outer electrons, a corresponding number of electrons from the unfinished penultimate layer can also participate in the formation of bonds. The most stable oxidation states for Cr are +3 and +6, Mo and W +6. Compounds in higher oxidation states are usually covalent and acidic in nature, much like the corresponding sulfur compounds. As the oxidation state decreases, the acidic character of the compounds weakens.

In the series Cr - Mo - W, the ionization energy increases, i.e. the electron shells of atoms become denser, especially strongly during the transition from Mo to W. Tungsten, due to lanthanide compression, has atomic and ionic radii close to those of Mo. Therefore, Mo and W are closer in properties to each other than to Cr.

Cr, Mo and W are white shiny metals. They are very hard (scratch glass) and refractory. Modifications of Cr, Mo and W, which are stable under normal conditions, have the structure of a body-centered cube. Tungsten is the most refractory of metals. In the Cr – Mo – W series, an increase in the melting temperature and heat of atomization (sublimation) is observed, which is explained by the strengthening of the covalent bond in the metal crystal arising due to d-electrons.

Although Cr, Mo and W are in the stress series before hydrogen, they are little susceptible to corrosion due to the formation of an oxide film on the surface. At room temperature these metals are slightly reactive.

Cr, Mo and W do not form stoichiometric compounds with hydrogen, but when heated they absorb it in significant quantities to form solid solutions. However, upon cooling, the absorbed hydrogen (especially in Mo and W) is partially released. As in other subgroups d-elements, with an increase in the ordinal number of an element in the Cr-Mo-W series, the chemical activity decreases. Thus, chromium displaces hydrogen from dilute HCl and H2SO4, while tungsten dissolves only in a hot mixture of hydrofluoric and nitric acids:

E o + 2HNO 3 + 8HF = H 2 [E +6 F 8 ] + 2NO + 4H 2 O

Due to the formation of anionic complexes EO 4 2-molybdenum and tungsten also interact when alloyed with alkalis in the presence of an oxidizing agent:

E o + 3NaN +5 O 3 + 2NaOH = Na 2 E +6 O 4 + 3NaN +3 O 2 + H 2 O

In concentrated HNO 3 and H 2 SO 4 chromium is passivated.

Cr, Mo and W form numerous compounds with S, Se, N, P, As, C, Si, B and other non-metals. The most interesting are the carbides: Cr 3 C 2, MoC, W 2 C, WC, which are second only to diamond in hardness and have high melting points, are used for the manufacture of especially hard alloys.

In direct interaction with halogens, chromium forms only di-, tri- and tetrahalides, and molybdenum and tungsten - and higher - penta- and hexahalides. Most element halides in lower oxidation states are strong reducing agents and easily form complex compounds. Mo and W diamides are cluster-type compounds with MeMe bonds. Halides of elements in higher oxidation states are, as a rule, volatile compounds with covalent bonds that easily hydrolyze in water, usually with the formation of oxohalides:

MoCl 5 + H 2 O  MoOCl 3 + 2HCl

Elements of the chromium subgroup form numerous oxide compounds corresponding to the main oxidation states. All oxides under normal conditions are solids. For chromium, the most stable is Cr 2 O 3, and for Mo and W – MoO 3 and WO 3. In the Cr - W series, the thermodynamic stability of acidic oxides EO 3 increases. Lower oxides are strong reducing agents and exhibit a basic character. An increase in the degree of oxidation is accompanied by an increase in acidic properties. Thus, Cr 2 O 3 is an amphoteric oxide, and CrO 3 (EO 3) is a typical acid oxide with the properties of a strong oxidizing agent. The only highly soluble oxide - CrO 3 - when dissolved in water, forms chromic acid:

CrO 3 + H 2 O  H 2 CrO 4 .

MoO 3 and WO 3 are poorly soluble in water and their acidic nature manifests itself when dissolved in alkalis:

2KON + EO 3  K 2 EO 4 + H 2 O.

Of the hydroxides of the E(OH) 2 type, only the poorly soluble base Cr(OH) 2 is known, which is formed when solutions of Cr 2+ salts are treated with alkalis. Cr(OH) 2 and Cr 2+ salts are strong reducing agents that are easily oxidized by atmospheric oxygen and even water to Cr 3+ compounds. Mo 2+ and W 2+ hydroxides are not released due to their instant oxidation with water.

The gray-blue hydroxide Cr(OH) 3 precipitated from solutions of Cr 3+ salts has a variable composition Cr 2 O 3  n H 2 O. This is a layered multinuclear polymer in which the role of ligands is played by OH - and OH 2, and the role of bridges is played by OH - groups.

Its composition and structure depend on the conditions of production. Freshly obtained Cr(OH) 3 is highly soluble in acids and alkalis, which cause the rupture of bonds in the layered polymer:

3+  Cr(OH) 3  3-

Mo(OH) 3, which is poorly soluble in water and acids, is obtained by treating Mo 3+ compounds with alkalis or ammonia. It is a strong reducing agent (decomposes water releasing hydrogen). The best known are the hydroxide derivatives Cr +6, Mo +6 and W +6. These are, first of all, acids of the type H 2 EO 4 and H 2 E 2 O 7 and their corresponding salts. Chromic H 2 CrO 4 and dichromic H 2 Cr 2 O 7 acids are of medium strength and exist only in aqueous solutions, but the salts corresponding to them are yellow chromates (CrO 4 2- anion) and orange dichromates (Cr 2 O 7 2- anion) , are stable and can be isolated from solutions.

The mutual transitions of chromate and dichromate can be expressed by the equation:

2CrO 4 2- + 2H +  2HCrO 4 -  Cr 2 O 7 2- + H 2 O

Chromates and dichromates are strong oxidizing agents. Molybdic and tungstic acids are slightly soluble in water. When alkalis act on H 2 MoO 4 (H 2 WO 4), or when MoO 3 (WO 3) melts with alkalis, depending on the ratio of the amounts of reagents, molybdates (tungstates) or isopolymolybdates (isopolytungstates) are formed:

MoO 3 + 2NaOH  Na 2 MoO 4 + H 2 O

3MoO 3 + NaOH  Na 2 Mo 3 O 10 + H 2 O

Isopolycompounds Mo +6 have different compositions: M 2 + Mo n O 3 n +1 (n=2, 3, 4); M 6 + Mon O 3 n +3 (n = 6, 7); M 4 + Mo 8 O 26. The tendency to polymerize from chromium to tungsten increases. Mo and W are characterized by the formation of heteropolyacids, i.e. polyacids containing in the anion, in addition to oxygen and molybdenum (tungsten), another element: P, Si, B, Te, etc. Heteropolycompounds are formed by acidifying a mixture of salts and mixing the corresponding acids, for example:

12Na 2 EO 4 + Na 2 SiO 3 + 22HNO 3  Na 4 + 22NaNO 3 + 11H 2 O.

Cr +6, Mo +6, and W +6 are characterized by the formation of peroxo compounds. Peroxide CrO 5 is known, having the structure CrO(O 2) 2. This unstable dark blue compound, existing in solutions, is obtained by treating solutions of chromates or dichromates with diethyl ether and a mixture of H 2 O 2 and H 2 SO 4. This reaction detects chromium (Cr +6) even in small quantities. Peroxochromates K[(Cr(O 2) 2 O)OH)] H 2 O, M 3, M= Na, K, NH 4 + were obtained.