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| {{short description|The element iron in its +2 oxidation state}} | |||
| ] | |||
| {{R with history}} | |||
| In ], '''iron(II)''' refers to the ] ] in its +2 ]. The adjective '''''ferrous''''' or the prefix '''''ferro-''''' is often used to specify such compounds, as in ''ferrous chloride'' for ] ({{chem2|FeCl2}}). The adjective '']'' is used instead for ] salts, containing the cation Fe<sup>3+</sup>. The word '']'' is derived from the ] word {{wikt-lang|la|ferrum}}, meaning "iron". | |||
| In ] (salts), such an atom may occur as a separate ] (positive ion) abbreviated as '''Fe<sup>2+</sup>''', although more precise descriptions include other ligands such as water and halides. Iron(III) centres occur in ]es, such as in the ] ], {{chem2|(4-)}}, where six ] ligands are bound the metal centre; or, in ]s, such as the ] {{chem2|}}, where two ] anions are bound to the Fe<sup>II</sup> centre. | |||
| ==Iron(III) in biology== | |||
| 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 = ] | accessdate = 9 May 2022}}</ref> Many ]s in living beings contain iron(III) centers. Examples of such ]s include ], ], and the ]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 ]. 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 (]ated) environment, especially in ]s. ] and ]es can thrive in such environments by secreting compounds called ]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 ] 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> | |||
| In contrast to iron(III) aquo complexes, iron(II) aquo complexes are soluble in water near neutral pH. Ferrous is however oxidized by the oxygen in air, converting to iron(III). | |||
| ] of aqueous iron]] | |||
| ==Iron(II) salts and complexes== | |||
| <sup>2+</sup>.]] | |||
| Typically iron(II) salts, like the "]" are ]es with the formulas {{chem2|(2+), as found in ferrous ammonium sulfate.<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 ] to give the blue iron(II) derivative: | |||
| When metallic iron (oxidation state 0) is placed in a solution of ], iron(II) chloride is formed, with release of ] gas, by the reaction | |||
| : {{chem2|Fe^{0} + 2 H(+) -> Fe(2+) + H2}} | |||
| ] is oxidized by hydrogen peroxide to ], forming a ] and a ] in the process. This is the ]. Iron(III) is then reduced back to iron(II) by another molecule of hydrogen peroxide, forming a ] radical and a ]. The net effect is a ] of hydrogen peroxide to create two different oxygen-radical species, with water (H<sup>+</sup> + 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}}}} | |||
| {{NumBlk|:| 2 H<sub>2</sub>O<sub>2</sub> → HO<sup>•</sup> + HOO<sup>•</sup> + H<sub>2</sub>O|{{EquationRef|net reaction: 1+2}}}} | |||
| The ]s generated by this process engage in secondary reactions because the hydroxyl is a powerful, non-selective oxidant. | |||
| ==Iron(II) minerals and other solids== | |||
| ] | |||
| 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 ]. For example, "ferrous sulfide" can refer to the 1:1 species (mineral name ]) or a host of Fe-deficient derivatives (]). The mineral ] ("lode stone") is a mixed-valence compound with both Fe(II) and Fe(III), Fe<sub>3</sub>O<sub>4</sub>. | |||
| ==Bonding== | |||
| 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 so-called "strong field ligands" such as ], the six electrons pair up. Thus ] ({{chem2|(4-)}} has no unpaired electrons. It is low-spin. With so-called "weak field ligands" such as ], the four of the six electrons are unpaired. Thus ] ({{chem2|(2+)}} is paramagnetic. It is high-spin. With chloride, iron(III) forms tetrahedral complexes, e.g. ({{chem2|(2-)}}. Tetrahedral complexes are high spin. | |||
| ==See also== | |||
| * {{annotated link|Ferric chloride}} (]) | |||
| * {{annotated link|Ferric oxide}} (]) | |||
| * {{annotated link|Ferric fluoride}} (]) | |||
| * {{annotated link|Ferrous}} | |||
| ==References== | |||
| {{reflist}} | |||
| ] | |||
| ] | |||
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