Editing Chromium (section)

=== Common oxidation states ===
{|class="wikitable" style="float:right; margin-left:1em"
|-
! colspan=2|Oxidation <br/>states<ref group=note>Most common oxidation states of chromium are in bold. The right column lists a representative compound for each oxidation state.</ref><ref name="Greenwood">{{Greenwood&Earnshaw2nd}}</ref>
|-
| −4 (d<sup>10</sup>)
|Na<sub>4</sub>[Cr(CO)<sub>4</sub>]<ref>{{Citation |last1=Theopold |first1=Klaus H. |title=Chromium: Organometallic Chemistry |date=2011-12-15 |encyclopedia=Encyclopedia of Inorganic and Bioinorganic Chemistry |pages=eibc0042 |editor-last=Scott |editor-first=Robert A. |publisher=John Wiley & Sons, Ltd |language=en |doi=10.1002/9781119951438.eibc0042 |isbn=978-1-119-95143-8 |last2=Kucharczyk |first2=Robin R.}}.</ref>
|-
| −2 (d<sup>8</sup>) ||{{chem|Na|2|[Cr(CO)|5|]}}
|-
| −1 (d<sup>7</sup>) ||{{chem|Na|2|[Cr|2|(CO)|10|]}}
|-
| 0 (d<sup>6</sup>) ||[[bis(benzene)chromium|{{chem|Cr(C|6|H|6|)|2}}]]
|-
| +1 (d<sup>5</sup>) ||{{chem| K|3|[Cr(CN)|5|NO]}}
|-
| '''+2 (d<sup>4</sup>)'''||[[Chromium(II) chloride|{{chem|CrCl|2}}]]
|-
| '''+3 (d<sup>3</sup>)''' || [[Chromium(III) chloride|{{chem|CrCl|3}}]]
|-
| +4 (d<sup>2</sup>) ||{{chem|K|2|CrF|6}}
|-
| +5 (d<sup>1</sup>) ||[[Potassium tetraperoxochromate(V)|{{chem|K|3|Cr(O|2|)|4|}}]]
|-
| '''+6 (d<sup>0</sup>)''' || [[Potassium chromate|{{chem|K|2|CrO|4}}]]
|}

==== Chromium(0) ====
Many Cr(0) complexes are known.  [[Bis(benzene)chromium]] and [[chromium hexacarbonyl]] are highlights in [[organochromium chemistry]].

==== Chromium(II) ====
[[File:Chromium carbide Cr3C2.JPG|thumb|upright|[[Chromium(II) carbide]] (Cr<sub>3</sub>C<sub>2</sub>)]]
Chromium(II) compounds are uncommon, in part because they readily oxidize to chromium(III) derivatives in air.  Water-stable [[chromium(II) chloride]] {{chem|CrCl|2}} that can be made by reducing chromium(III) chloride with zinc. The resulting bright blue solution created from dissolving chromium(II) chloride is stable at neutral [[pH]].<ref name="HollemanAF" /> Some other notable chromium(II) compounds include [[chromium(II) oxide]] {{chem|CrO}}, and [[chromium(II) sulfate]] {{chem|CrSO|4}}. Many chromium(II) carboxylates are known. The red [[chromium(II) acetate]] (Cr<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub>) is somewhat famous. It features a Cr-Cr [[quadruple bond]].<ref>{{cite book|last1=Cotton|first1=FA|author-link1=F. Albert Cotton|last2=Walton|first2=RA|title=Multiple Bonds Between Metal Atoms|url=https://archive.org/details/multiplebondsbet0000cott|url-access=registration|publisher=Oxford University Press|location=Oxford|date=1993|isbn=978-0-19-855649-7}}</ref>

==== Chromium(III) ====
[[File:Chromium(III)-chloride-purple-anhydrous-sunlight.jpg|thumb|upright|Anhydrous chromium(III) chloride (CrCl<sub>3</sub>)]]
A large number of chromium(III) compounds are known, such as [[chromium nitrate|chromium(III) nitrate]], [[chromium(III) acetate]], and [[chromium(III) oxide]].<ref>{{cite web|title = Chromium(III) compounds|url = http://www.npi.gov.au/resource/chromium-iii-compounds|website = National Pollutant Inventory|publisher = Commonwealth of Australia|access-date = 8 November 2018|archive-date = 22 April 2021|archive-url = https://web.archive.org/web/20210422214943/http://www.npi.gov.au/resource/chromium-iii-compounds|url-status = live}}</ref> Chromium(III) can be obtained by dissolving elemental chromium in acids like [[hydrochloric acid]] or [[sulfuric acid]], but it can also be formed through the reduction of chromium(VI) by [[cytochrome]] [[cytochrome c|c7]].<ref>{{cite journal|last1 = Assfalg|first1 = M|last2 = Banci|first2 = L|last3 = Bertini|first3 = I|last4 = Bruschi|first4 = M|last5 = Michel|first5 = C|last6 = Giudici-Orticoni|first6 = M|last7 = Turano|first7 = P|title = NMR structural characterization of the reduction of chromium(VI) to chromium(III) by cytochrome c7|journal = Protein Data Bank|date = 31 July 2002|issue = 1LM2|doi = 10.2210/pdb1LM2/pdb|url = https://www.rcsb.org/structure/1lm2|access-date = 8 November 2018|archive-date = 2 October 2021|archive-url = https://web.archive.org/web/20211002124813/https://www.rcsb.org/structure/1lm2|url-status = live}}</ref> The {{chem|Cr|3+}} ion has a similar radius (63&nbsp;[[picometer|pm]]) to {{chem|Al|3+}} (radius 50&nbsp;pm), and they can replace each other in some compounds, such as in [[chrome alum]] and [[alum]].

Chromium(III) tends to form [[octahedral molecular geometry|octahedral]] complexes. Commercially available [[chromium(III) chloride]] hydrate is the dark green complex [CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]Cl. Closely related compounds are the pale green [CrCl(H<sub>2</sub>O)<sub>5</sub>]Cl<sub>2</sub> and violet [Cr(H<sub>2</sub>O)<sub>6</sub>]Cl<sub>3</sub>. If anhydrous violet<ref>{{cite book|first=George W.|last=Luther|date=2016|department=Hydrate (Solvate) Isomers|chapter=Introduction to Transition Metals|chapter-url=https://books.google.com/books?id=Fz7hCgAAQBAJ&pg=PA244|title=Inorganic Chemistry for Geochemistry & Environmental Sciences: Fundamentals & Applications|page=244|isbn=978-1-118-85137-1|publisher=John Wiley & Sons|access-date=2019-08-07|archive-date=10 June 2024|archive-url=https://web.archive.org/web/20240610050327/https://books.google.com/books?id=Fz7hCgAAQBAJ&pg=PA244#v=onepage&q&f=false|url-status=live}}</ref> [[chromium(III) chloride]] is dissolved in water, the violet solution turns green after some time as the chloride in the inner [[coordination sphere]] is replaced by water. This kind of reaction is also observed with solutions of [[chrome alum]] and other water-soluble chromium(III) salts. A [[Tetrahedral molecular geometry|tetrahedral]] coordination of [[chromium(III)]] has been reported for the Cr-centered [[Keggin structure|Keggin]] anion [α-CrW<sub>12</sub>O<sub>40</sub>]<sup>5–</sup>.<ref>{{Cite journal|last1=Gumerova|first1=Nadiia I.|last2=Roller|first2=Alexander|last3=Giester|first3=Gerald|last4=Krzystek|first4=J.|last5=Cano|first5=Joan|last6=Rompel|first6=Annette|date=2020-02-19|title=Incorporation of CrIII into a Keggin Polyoxometalate as a Chemical Strategy to Stabilize a Labile {CrIIIO4} Tetrahedral Conformation and Promote Unattended Single-Ion Magnet Properties|journal=Journal of the American Chemical Society|volume=142|issue=7|pages=3336–3339|doi=10.1021/jacs.9b12797|issn=0002-7863|pmc=7052816|pmid=31967803}}</ref>

[[Chromium(III) hydroxide]] (Cr(OH)<sub>3</sub>) is [[amphoterism|amphoteric]], dissolving in acidic solutions to form [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>, and in basic solutions to form {{chem|[Cr(OH)|6|]|3-}}. It is dehydrated by heating to form the green chromium(III) oxide (Cr<sub>2</sub>O<sub>3</sub>), a stable oxide with a crystal structure identical to that of [[corundum]].<ref name="HollemanAF" />

==== Chromium(VI) ====
{{Main|Hexavalent chromium}}
[[Hexavalent chromium|Chromium(VI) compounds]] are oxidants at low or neutral pH. [[chromate and dichromate|Chromate]] anions ({{chem|CrO|4|2-}}) and [[chromate and dichromate|dichromate]] (Cr<sub>2</sub>O<sub>7</sub><sup>2−</sup>) anions are the principal ions at this oxidation state. They exist at an equilibrium, determined by pH:
:2 [CrO<sub>4</sub>]<sup>2−</sup> + 2 H<sup>+</sup> {{eqm}} [Cr<sub>2</sub>O<sub>7</sub>]<sup>2−</sup> + H<sub>2</sub>O
Chromium(VI) oxyhalides are known also and include [[chromyl fluoride]] (CrO<sub>2</sub>F<sub>2</sub>) and [[chromyl chloride]] ({{chem|CrO|2|Cl|2}}).<ref name="HollemanAF">{{Cite book|publisher = Walter de Gruyter|date = 1985|edition = 91–100|pages = 1081–1095|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|last1 = Holleman|first1 = Arnold F|last2 = Wiber|first2 = Egon|last3 = Wiberg|first3 = Nils|chapter = Chromium|language = de}}</ref>  However, despite several erroneous claims, [[chromium hexafluoride]] (as well as all higher hexahalides) remains unknown, as of 2020.<ref>{{Cite journal|last=Seppelt|first=Konrad|date=2015-01-28|title=Molecular Hexafluorides|journal=Chemical Reviews|language=en|volume=115|issue=2|pages=1296–1306|doi=10.1021/cr5001783|pmid=25418862|issn=0009-2665}}</ref>

[[File:Chrom(VI)-oxid.jpg|thumb|right|upright|Chromium(VI) oxide]]
[[Sodium chromate]] is produced industrially by the oxidative roasting of [[chromite]] ore with [[sodium carbonate]]. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of [[potassium chromate]]. At yet lower pH values, further condensation to more complex [[oxyanion]]s of chromium is possible.

Both the [[chromate and dichromate]] anions are strong oxidizing reagents at low pH:<ref name="HollemanAF" />
:{{chem|Cr|2|O|7|2-}} + 14 {{chem|H|3|O|+}} + 6 e<sup>−</sup> → 2 {{chem|Cr|3+}} + 21 {{chem|H|2|O}} (ε<sub>0</sub> = 1.33&nbsp;V)

They are, however, only moderately oxidizing at high pH:<ref name="HollemanAF" />
:{{chem|CrO|4|2-}} + 4 {{chem|H|2|O}} + 3 e<sup>−</sup> → {{chem|Cr(OH)|3}} + 5 {{chem|OH|-}} (ε<sub>0</sub> = −0.13&nbsp;V)

[[File:Chroman sodný.JPG|thumb|upright|[[Sodium chromate]] (Na<sub>2</sub>CrO<sub>4</sub>)]]
Chromium(VI) compounds in solution can be detected by adding an acidic [[hydrogen peroxide]] solution. The unstable dark blue [[chromium(VI) peroxide]] (CrO<sub>5</sub>) is formed, which can be stabilized as an ether adduct {{chem|CrO|5|·OR|2}}.<ref name="HollemanAF" />

[[Chromic acid]] has the hypothetical formula {{chem|H|2|CrO|4}}. It is a vaguely described chemical, despite many well-defined chromates and dichromates being known. The dark red [[chromium(VI) oxide]] {{chem|CrO|3}}, the acid [[anhydride]] of chromic acid, is sold industrially as "chromic acid".<ref name="HollemanAF" /> It can be produced by mixing sulfuric acid with dichromate and is a strong oxidizing agent.