Chemistry of Chromium
Chromium (Cr) is the 24th element of periodic table in transition metal series. Cr is the first element of 6th(B)
group and has the electronic configuration of [Ar]3d54s. Chromium metal is hard, shiny, brittle and white with
high melting point of 1907 °C and extreme corrosion resistance. Chromium has vast importance in industrial
use, the most important application being chrome plating for corrosion resistance and steel alloys of chromium
for stainless steel. In nature chromium can be found in chromite-ore FeCr2O4 and it is 22nd most abundant
element on earth’s crust. Chromite has a spinel structure with iron(II) on tetrahedral and chromium(III) on the
octahedral sites. The largest chromite producers are South-Africa, Kazakhstan and India. Chromium has high
range of oxidation states the lowest being (–II) and the highest corresponding to the number of 3d and 4s
electrons, six (VI). The most stable states are states II and III and with state VI also being the most important
ones.
Chromium is reactive towards halogens, silicon, sulfur, boron, carbon and oxygen at elevated temperatures. It
can be dissolved in nonoxidizing mineral acids fairly easily, but not in nitric acid because of metal passivation.
Chromium has wide range of binary compounds such as halides, oxides and sulfides in oxidation states from II
to VI. Anhydrous halides can be obtained by reactions with HF, HCl, HBr or I2 and with chromium metal at 700
°C. Important halides are for example CrIICl2 and CrIIICl3. Chromium(II) chloride is very hygroscopic and usually
green colored. CrCl2 is used in synthesis of inorganic and organometallic reagents such as Nozaki–Hiyama–Kishi
reaction, in which it acts as a reducing agent. On the other hand CrCl3 has an ability to form adducts with
tetrahydrofuran (THF). Because of its solubility in organic solvents, these furunates are useful in making of
organochromium compounds.
Almost all of the chromium(III) complexes are hexacoordinate with octahedral structure. They are known for
their kinetic stability and that is why thousands of them have been isolated. Well known complexes are for
example Chromium(III) chloride hexahydrate and amine complexes. Important chromium(VI) compounds
chromate (CrO42-) and dichromate (Cr2O72-) have strong oxidizing capabilities. They are important oxidizers in
organic chemistry but are also used in inorganic quantitative analysis.
Important oxides of chromium are CrIII2O3, CrIIO2 and CrVIO3. Cr2O3 is hard and brittle and it can be produced in
various way, for example, by burning chromium in oxygen or thermally decomposing ammonium dichromate at
approximately 200 °C. Cr2O3 is amphoteric and insoluble to water but dissolves in acids forming aqua ions
[Cr(H2O)6]3+. Chromium(III)oxide is commonly used as green pigment, viridian in paints, glasses and inks as well
as polishing agent for knives etc. CrO2 is made from hydrothermal reduction of CrO3 at 800 K and 2000 bars.
Chromium(IV)oxide has distinctive ferromagnetic and conductive properties because of its overlapping
chromium d and oxygen pπ orbitals. CrO2 has seen use as coatings on magnetic memory devices such as high
quality audiocassette tapes. Chromium(VI)oxide is produced by adding sulfuric acid in dichromate solution. In
CrO3 is used as a very powerful oxidizer and has wide range of applications such as in chromium electroplating,
reagent for chromium dioxide, wood protection and manufacturing catalysts. Chromium(VI) oxides, as all other
hexavalent chromium species, are notorious for their corrosiveness and acute toxicity for humans and the
environment. That’s why they’ve been partly displaced by Chromium(III) derivatives in chrome electroplating.
As stated earlier, chromium has a huge number on different applications. The most important are metallurgical
alloying of steel and chromium plating by electroplating. Approximately 85% of produced chromium is used in
those industrial areas. [wiki]Owing to their high melting point chromite and chromium(III) has also seen use as
refractory material in furnaces, molds etc. In addition to chromium(III)oxide’s green pigments the one of the
most used yellow pigments, chrome yellow, is made from lead(II)chromate PbCrO4, although its popularity has
diminished in recent years because of its carcinogenicity. Chromium compounds are also actively used in
catalysis materials. Chromium oxide and chromium with aluminum(III)oxide support are widely used as catalyst
in various organic syntheses. Also, chromium(III)- and chromium(VI)oxides are utilized as a starting material for
important Phillips catalyst and chromocene, which are important catalysts in polyethylene production.
SOURCES:
[1] Cotton & Wilkinson: Advanced Inorganic Chemistry 5th ed.
[2] CRC Handbook of Chemistry and Physics 96th ed.
[3] Chen, H., Dawe, L. N. & Kozak, C. M. Chromium(iii) amine-bis(phenolate) complexes as catalysts for
copolymerization of cyclohexene oxide and CO2. Catal. Sci. Technol. 4, 1547 (2014).
[3] MacAdams, L. A., Buffone, G. P., Incarvito, C. D., Rheingold, A. L. & Theopold, K. H. A chromium catalyst for
the polymerization of ethylene as a homogeneous model for the phillips catalyst. J. Am. Chem. Soc. 127, 1082–
3 (2005).
[4] Buchel, Moretto, Woditsch: Industrial Inorganic Chemistry 2nd ed.
[5] https://en.wikipedia.org/wiki/Chromium(III)_oxide
[6] https://en.wikipedia.org/wiki/Chromium