View source for Ferrous

{{short description|Iron in its +2 oxidation state}}
{{For|ferrous metals and alloys|Ferrous metallurgy}}
{{Redirect|Reduced iron|the elemental powder|Direct reduced iron}}

[[File:Eisen(II)-chlorid-Tetrahydrat.jpg|thumb|Iron(II) chloride tetrahydrate, {{chem2|FeCl2*4H2O}}.]]

In [[chemistry]], '''iron(II)''' refers to the [[chemical element|element]] [[iron]] in its +2 [[oxidation number|oxidation state]]. The adjective '''''ferrous''''' or the prefix '''''ferro-''''' is often used to specify such compounds, as in ''ferrous chloride'' for [[iron(II) chloride]] ({{chem2|FeCl2}}). The adjective ''[[ferric]]'' is used instead for [[iron(III)]] salts, containing the cation Fe<sup>3+</sup>. The word ''[[wikt:ferrous|ferrous]]'' is derived from the [[Latin]] word {{wikt-lang|la|ferrum}}, meaning "iron".

In [[salt (chemistry)|ionic compounds]] (salts), such an atom may occur as a separate [[cation]] (positive ion) abbreviated as '''Fe<sup>2+</sup>''', although more precise descriptions include other ligands such as water and halides. Iron(II) centres occur in [[coordination complex]]es, such as in the [[anion]] [[ferrocyanide]], {{chem2|[Fe(CN)6](4-)}}, where six [[cyanide]] ligands are bound the metal centre; or, in [[organometallic compound]]s, such as the [[ferrocene]] {{chem2|[Fe(C5H5)2]}}, where two [[cyclopentadienyl]] anions are bound to the Fe<sup>II</sup> centre.

==Ferrous ions in biology==
{{main article|Iron in biology}}
{{See also|Iron metabolism}}
All known forms of life require iron.<ref>{{cite web | title = Iron integral to the development of life on Earth – and the possibility of life on other planets | url = https://www.ox.ac.uk/news/2021-12-07-iron-integral-development-life-earth-and-possibility-life-other-planets | date = 7 December 2021 | publisher = [[University of Oxford]] | accessdate = 9 May 2022}}</ref> Many [[protein]]s in living beings contain iron(II) centers. Examples of such [[metalloprotein]]s include [[hemoglobin]], [[ferredoxin]], and the [[cytochrome]]s. In many of these proteins, Fe(II) converts reversibly to Fe(III).<ref>{{cite book |author=Berg, Jeremy Mark |author2=Lippard, Stephen J. |title=Principles of bioinorganic chemistry |publisher=University Science Books |location=Sausalito, Calif |year=1994 |isbn=0-935702-73-3 }}</ref>

Insufficient iron in the human diet causes [[anemia]]. Animals and humans can obtain the necessary iron from foods that contain it in assimilable form, such as meat. Other organisms must obtain their iron from the environment. However, iron tends to form highly insoluble iron(III) oxides/hydroxides in aerobic ([[oxygen]]ated) environment, especially in [[calcareous soil]]s. [[Bacteria]] and [[graminaceae|grass]]es can thrive in such environments by secreting compounds called [[siderophore]]s that form soluble complexes with iron(III), that can be reabsorbed into the cell. (The other plants instead encourage the growth around their roots of certain bacteria that [[redox|reduce]] iron(III) to the more soluble iron(II).)<ref name=marsch94>H. Marschner and V. Römheld (1994): "Strategies of plants for acquisition of iron". ''Plant and Soil'', volume 165, issue 2, pages 261–274. {{doi|10.1007/BF00008069}}</ref>[[File:Pourbaix Diagram of Iron.svg|thumb|[[Pourbaix diagram]] of aqueous iron]]In contrast to iron(III) aquo complexes, iron(II) aquo complexes are soluble in water near neutral pH.{{citation needed|date=April 2024}} Ferrous iron is, however, oxidized by the oxygen in air, converting to iron(III).<ref>{{cite encyclopedia|last=Petsch|first=S.T.|title=Treatise on Geochemistry|edition=Second|year=2014|volume=10|series=Reference Module in Earth Systems and Environmental Sciences|pages=437–473|isbn=978-0-08-095975-7|chapter=10.11 - The Global Oxygen Cycle|doi=10.1016/B978-0-08-095975-7.00811-1|editor-last1=Holland|editor-first1=H.D.|editor-last2=Turekian|editor-first2=K.K.|publisher=Elsevier}}</ref>

==Ferrous salts and complexes==
Typically iron(II) salts, like the "[[iron(II) chloride|chloride]]" are [[aquo complex]]es with the formulas {{chem2|[Fe(H2O)6](2+)}}, as found in [[Ammonium iron(II) sulfate|Mohr's salt]].<ref name="earn">{{cite book |author=Earnshaw, A. |author2=Greenwood, N. N. |title=Chemistry of the elements |publisher=Butterworth-Heinemann |location=Oxford |year=1997 |isbn=0-7506-3365-4 |edition=2nd}}</ref> 

The aquo ligands on iron(II) complexes are labile. It reacts with [[1,10-Phenanthroline|1,10-phenanthroline]] to give the blue iron(II) derivative:

When metallic iron (oxidation state 0) is placed in a solution of [[hydrochloric acid]], iron(II) chloride is formed, with release of [[hydrogen]] gas, by the reaction
: {{chem2|Fe^{0} + 2 H(+) -> Fe(2+) + H2}}

Iron(II) is oxidized by hydrogen peroxide to [[iron(III)]], forming a [[hydroxyl radical]] and a [[hydroxide ion]] in the process. This is the [[Fenton reaction]]. Iron(III) is then reduced back to iron(II) by another molecule of hydrogen peroxide, forming a [[hydroperoxyl]] radical and a [[hydrogen atom|proton]]. The net effect is a [[disproportionation]] of hydrogen peroxide to create two different oxygen-radical species, with water (H<sup>+</sup>&nbsp;+&nbsp;OH<sup>−</sup>) as a byproduct.<ref>{{cite journal |doi=10.1021/acs.chemrev.0c00977 |title=Biomedicine Meets Fenton Chemistry |date=2021 |last1=Tang |first1=Zhongmin |last2=Zhao |first2=Peiran |last3=Wang |first3=Han |last4=Liu |first4=Yanyan |last5=Bu |first5=Wenbo |journal=Chemical Reviews |volume=121 |issue=4 |pages=1981–2019 |pmid=33492935 |s2cid=231712587 }}</ref> 

{{NumBlk|:| Fe<sup>2+</sup> + H<sub>2</sub>O<sub>2</sub> → Fe<sup>3+</sup> + HO<sup>•</sup> + OH<sup>−</sup>|{{EquationRef|1}}}}
{{NumBlk|:| Fe<sup>3+</sup> + H<sub>2</sub>O<sub>2</sub> → Fe<sup>2+</sup> + HOO<sup>•</sup> + H<sup>+</sup>|{{EquationRef|2}}}}

The [[free radical]]s generated by this process engage in secondary reactions, which can degrade many organic and biochemical compounds.[[File:Fe(bipy)3 redox.svg|thumb|360px|center|Redox reaction of [Fe(bipyridine)<sub>3</sub>]<sup>2+</sup>.]]

==Ferrous minerals and other solids==
[[File:Iron(II) oxide.jpg|thumb|Iron(II) oxide (ferrous oxide), FeO, is a very complicated material that contains iron(II).]]

Iron(II) is found in many minerals and solids. Examples include the sulfide and oxide, FeS and FeO. These formulas are deceptively simple because these sulfides and oxides are often [[nonstoichiometric]]. For example, "ferrous sulfide" can refer to the 1:1 species (mineral name [[troilite]]) or a host of Fe-deficient derivatives ([[pyrrhotite]]). The mineral [[magnetite]] ("lode stone") is a mixed-valence compound with both Fe(II) and Fe(III), Fe<sub>3</sub>O<sub>4</sub>.

==Bonding==
[[File:L.s. vs h.s. d6 octahedral.svg|thumb|d-orbital splitting scheme for low- and high spin octahedral Fe(II) complexes.]]Iron(II) is a d<sup>6</sup> center, meaning that the metal has six "valence" electrons in the 3d orbital shell. The number and type of ligands bound to iron(II) determine how these electrons arrange themselves. With the so-called "strong field ligands" such as [[cyanide]], the six electrons pair up. Thus [[ferrocyanide]] ({{chem2|[Fe(CN)6](4-)}} has no unpaired electrons, meaning it is a low-spin complex. With so-called "weak field ligands" such as [[water]], four of the six electrons are unpaired, meaning it is a [[High Spin Complex|high-spin]] complex. Thus [[aquo complex]] {{chem2|[Fe(H2O)6](2+)}} is [[paramagnetic]]. With [[chloride]], iron(II) forms tetrahedral complexes, e.g. {{chem2|[FeCl4](2-)}}. Tetrahedral complexes are high-spin complexes.

==Gallery==
{{Gallery
| title        = Selected Fe(II) compounds
| align        =
| footer       =
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| captionstyle =
| File:Iron(II) nitrate hexahydrate.jpg
 | alt2= 
 | [[Iron(II) nitrate|Ferrous nitrate hexahydrate]], Fe(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O
| File:Iron(II)-oxalate-sample.jpg
 | alt3=
 | [[Ferrous oxalate|Ferrous oxalate dihydrate]], [[Humboldtine]], FeC<sub>2</sub>O<sub>4</sub>·2H<sub>2</sub>O
| File:7314M-vivianite2.jpg
 | alt4=
 | [[Vivianite]], Ferrous phosphate octahydrate, Fe<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>·8H<sub>2</sub>O
| File:Ferrous_sulfate.jpg
 | alt5=
 | [[Iron(II) sulfate|Ferrous sulfate heptahydrate]], [[Melanterite]], FeSO<sub>4</sub>·7H<sub>2</sub>O
| File:Iron(II)-sulfide-sample.jpg
 | alt6=
 | [[Iron(II) sulfide|Ferrous sulfide]], [[Troilite]], FeS
| FeSiO3.png
 | Ferrous silicate, [[Ferrosilite]], FeSiO<sub>3</sub>
}}

==See also==
* {{annotated link|Ferromagnetism}}
* {{annotated link|Iron(II) oxide}} (ferrous oxide)
* {{annotated link|Iron(II) bromide}} (ferrous bromide)
* [[:Category:Iron(II) minerals]]

==References==

{{wiktionary|ferrous|nonferrous}}
{{reflist|30em}}

[[Category:Iron]]
[[Category:Iron(II) compounds| ]]