Ferric

Last updated March 11, 2024

In chemistry, Ferric refers to the element iron in its +3 oxidation state. Ferric chloride is an alternative name for iron(III) chloride (FeCl₃). The adjective ferrous is used instead for iron(II) salts, containing the cation Fe²⁺. The word ferric is derived from the Latin word ferrum , meaning "iron".

Although often abbreviated as Fe³⁺, that naked ion does not exist except under extreme conditions. Iron(III) centres are found in many compounds and coordination complexes, where Fe(III) is bonded to several ligands. A molecular ferric complex is the anion ferrioxalate, [Fe(C₂O₄)₃]³⁻, with three bidentate oxalate ions surrounding the Fe core. Relative to lower oxidation states, ferric is less common in organoiron chemistry, but the ferrocenium cation [Fe(C₂H₅)₂]⁺ is well known.

Iron(III) in biology

All known forms of life require iron, which usually exists in Fe(II) or Fe(III) oxidation states.^[1] Many proteins in living beings contain iron(III) centers. Examples of such metalloproteins include oxyhemoglobin, ferredoxin, and the cytochromes. Many organisms, from bacteria to humans, store iron as microscopic crystals (3 to 8 nm in diameter) of iron(III) oxide hydroxide, inside a shell of the protein ferritin, from which it can be recovered as needed. ^[2]

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 (oxygenated) environment, especially in calcareous soils. Bacteria and grasses can thrive in such environments by secreting compounds called siderophores 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 reduce iron(III) to the more soluble iron(II).)^[3]

The insolubility of iron(III) compounds is also responsible for the low levels of iron in seawater, which is often the limiting factor for the growth of the microscopic plants (phytoplankton) that are the basis of the marine food web.^[4]

Iron(III) salts and complexes

Typically iron(III) salts, like the "chloride" are aquo complexes with the formulas [Fe(H₂O)₅Cl]²⁺, [Fe(H₂O)₄Cl₂]⁺, and [Fe(H₂O)₃Cl₃]. Iron(III) nitrate dissolved in water to give [Fe(H₂O)₆]³⁺ ions. In these complexes, the protons are acidic. Eventually these solutions hydrolyze producing iron(III) hydroxide Fe(OH)₃ that further converts to polymeric oxide-hydroxide via the process called olation. These hydroxides precipitates out of the solution as solids. That reaction liberates hydrogen ions H⁺ lowering the pH of its solutions. The equilibria are elaborate:^[5]

[Fe(H₂O)₆]³⁺ ⇌ [Fe(H₂O)₅OH]²⁺ + H⁺

[Fe(H₂O)₅OH]²⁺ ⇌ [Fe(H₂O)₄(OH)₂]⁺ + H⁺

2 [Fe(H₂O)₄(OH)₂]⁺ ⇌ [Fe₂(H₂O)₈(OH)₂]+2 + 2 H₂O

Various chelating compounds prevent the polymerization. These same ligands can even dissolve iron(III) oxides and hydroxides. One of these ligands is EDTA, which is often used to dissolve iron deposits or added to fertilizers to make iron in the soil available (soluble) to plants. Citrate also solubilizes ferric ion at neutral pH, although its complexes are less stable than those of EDTA. Many chelating ligands - the siderophores - are produced naturally to dissolve iron(III) oxides.

The aquo ligands on iron(III) complexes are labile. This behavior is visualized by the color change brought about by reaction with thiocyanate:

[Fe(H₂O)₆]³⁺ + SCN⁻ ⇌ [Fe(SCN)(H₂O)₅]²⁺ + H₂O

Whereas [Fe(H₂O)₆]³⁺ is nearly colorless, [Fe(SCN)(H₂O)₅]²⁺ is deep red.

While iron(III) aquo complexes tend to convert to polymeric oxy-hydroxides, iron(III) complexes with other ligands form stable solutions. The complex with 1,10-phenanthrolinebipyridine is soluble and can sustain reduction to it iron(II) derivative:

Iron(III) minerals and other solids

Ferric oxide, commonly called rust, is a very complicated material that contains iron(III). Iron(III)-oxide-sample.jpg — Ferric oxide, commonly called rust, is a very complicated material that contains iron(III).

Iron(III) is found in many minerals and solids, e.g., oxide Fe₂O₃ (hematite) and iron(III) oxide-hydroxide FeO(OH) are extremely insoluble reflecting their polymeric structure Rust is a mixture of iron(III) oxide and oxide-hydroxide that usually forms when iron metal is exposed to humid air. Unlike the passivating oxide layers that are formed by other metals, like chromium and aluminum, rust flakes off, because it is bulkier than the metal that formed it. Therefore, unprotected iron objects will in time be completely turned into rust.

Bonding

Iron(III) is a d⁵ center, meaning that the metal has five "valence" electrons in the 3d orbital shell. The number and type of ligands bound to iron(III) determine how these electrons arrange themselves. With so-called "strong field ligands" such as cyanide, the five electrons pair up as best they can. Thus ferricyanide ([Fe(CN)₆]³⁻ has only one unpaired electron. It is low-spin. With so-called "weak field ligands" such as water, the five electrons are unpaired. Thus aquo complex ([Fe(H₂O)₆]³⁺ has only five unpaired electrons. It is high-spin. With chloride, iron(III) forms tetrahedral complexes, e.g. ([Fe(Cl)₄]⁻. Tetrahedral complexes are high spin. The magnetism of ferric complexes can show when they are high or low spin.

Related Research Articles

<span class="mw-page-title-main">Coordination complex</span> Molecule or ion containing ligands datively bonded to a central metallic atom

A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.

Hydroxide is a diatomic anion with chemical formula OH⁻. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. The corresponding electrically neutral compound HO^• is the hydroxyl radical. The corresponding covalently bound group –OH of atoms is the hydroxy group. Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry.

<span class="mw-page-title-main">Inorganic chemistry</span> Field of chemistry

Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture.

<span class="mw-page-title-main">Ferrous</span> The element iron in its +2 oxidation state

In chemistry, iron(II) refers to the element iron in its +2 oxidation state. The adjective ferrous or the prefix ferro- is often used to specify such compounds, as in ferrous chloride for iron(II) chloride (FeCl₂). The adjective ferric is used instead for iron(III) salts, containing the cation Fe³⁺. The word ferrous is derived from the Latin word ferrum, meaning "iron".

Iron(III) chloride describes the inorganic compounds with the formula FeCl₃(H₂O)_x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated forms which are both hygroscopic. They feature iron in its +3 oxidation state. The anhydrous derivative is a Lewis acid, while all forms are mild oxidizing agents. It is used as a water cleaner and as an etchant for metals.

<span class="mw-page-title-main">Iron(II) chloride</span> Chemical compound

Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl₂. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are often off-white. FeCl₂ crystallizes from water as the greenish tetrahydrate, which is the form that is most commonly encountered in commerce and the laboratory. There is also a dihydrate. The compound is highly soluble in water, giving pale green solutions.

<span class="mw-page-title-main">Metal ammine complex</span>

In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way for historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost all metal ions bind ammonia as a ligand, but the most prevalent examples of ammine complexes are for Cr(III), Co(III), Ni(II), Cu(II) as well as several platinum group metals.

<span class="mw-page-title-main">Iron(II) hydroxide</span> Chemical compound

Iron(II) hydroxide or ferrous hydroxide is an inorganic compound with the formula Fe(OH)₂. It is produced when iron(II) salts, from a compound such as iron(II) sulfate, are treated with hydroxide ions. Iron(II) hydroxide is a white solid, but even traces of oxygen impart a greenish tinge. The air-oxidised solid is sometimes known as "green rust".

Iron(III) oxide-hydroxide or ferric oxyhydroxide is the chemical compound of iron, oxygen, and hydrogen with formula FeO(OH).

<span class="mw-page-title-main">Iron(III) nitrate</span> Chemical compound

Iron(III) nitrate, or ferric nitrate, is the name used for a series of inorganic compounds with the formula Fe(NO₃)₃^.(H₂O)_n. Most common is the nonahydrate Fe(NO₃)₃^.(H₂O)₉. The hydrates are all pale colored, water-soluble paramagnetic salts.

Silver is a relatively unreactive metal, although it can form several compounds. The common oxidation states of silver are (in order of commonness): +1 (the most stable state; for example, silver nitrate, AgNO₃); +2 (highly oxidising; for example, silver(II) fluoride, AgF₂); and even very rarely +3 (extreme oxidising; for example, potassium tetrafluoroargentate(III), KAgF₄). The +3 state requires very strong oxidising agents to attain, such as fluorine or peroxodisulfate, and some silver(III) compounds react with atmospheric moisture and attack glass. Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidizing agent, krypton difluoride.

Iron shows the characteristic chemical properties of the transition metals, namely the ability to form variable oxidation states differing by steps of one and a very large coordination and organometallic chemistry: indeed, it was the discovery of an iron compound, ferrocene, that revolutionalized the latter field in the 1950s. Iron is sometimes considered as a prototype for the entire block of transition metals, due to its abundance and the immense role it has played in the technological progress of humanity. Its 26 electrons are arranged in the configuration [Ar]3d⁶4s², of which the 3d and 4s electrons are relatively close in energy, and thus it can lose a variable number of electrons and there is no clear point where further ionization becomes unprofitable.

Iron(III) sulfate (or ferric sulfate), is a family of inorganic compounds with the formula Fe₂(SO₄)₃(H₂O)_n. A variety of hydrates are known, including the most commonly encountered form of "ferric sulfate". Solutions are used in dyeing as a mordant, and as a coagulant for industrial wastes. Solutions of ferric sulfate are also used in the processing of aluminum and steel.

In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution. The phenomenon is important for understanding the relationship between metal aquo complexes and metal oxides, which are represented by many minerals.

<span class="mw-page-title-main">Iron(II) oxalate</span> Chemical compound

Ferrous oxalate (iron(II) oxalate) are inorganic compound with the formula FeC₂O₄(H₂O)_x where x is 0 or 2. These are orange compounds, poorly soluble in water.

In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [M(H₂O)_n]^z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand, but of course many complexes are known to consist of a mix of aquo and other ligands.

Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.

<span class="mw-page-title-main">Ferric EDTA</span> Chemical compound

Ferric EDTA is the coordination complex formed from ferric ions and EDTA. EDTA has a high affinity for ferric ions. It gives yellowish aqueous solutions.

<span class="mw-page-title-main">Iron(II) nitrate</span> Chemical compound

Iron(II) nitrate is the nitrate salt of iron(II). It is commonly encountered as the green hexahydrate, Fe(NO₃)₂·6H₂O, which is a metal aquo complex, however it is not commercially available unlike iron(III) nitrate due to its instability to air. The salt is soluble in water serves as a ready source of ferrous ions.

Cobalt compounds are chemical compounds formed by cobalt with other elements.

References

↑ "Iron integral to the development of life on Earth – and the possibility of life on other planets". University of Oxford. 7 December 2021. Retrieved 9 May 2022.
↑ Berg, Jeremy Mark; Lippard, Stephen J. (1994). Principles of bioinorganic chemistry. Sausalito, Calif: University Science Books. ISBN 0-935702-73-3.
↑ 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
↑ Boyd PW, Watson AJ, Law CS, et al. (October 2000). "A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization". Nature. 407 (6805): 695–702. Bibcode:2000Natur.407..695B. doi:10.1038/35037500. PMID 11048709. S2CID 4368261.
↑ Earnshaw, A.; Greenwood, N. N. (1997). Chemistry of the elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4.

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[1] "Iron integral to the development of life on Earth – and the possibility of life on other planets". University of Oxford. 7 December 2021. Retrieved 9 May 2022.

[2] Berg, Jeremy Mark; Lippard, Stephen J. (1994). Principles of bioinorganic chemistry. Sausalito, Calif: University Science Books. ISBN 0-935702-73-3.

[marsch94-3] 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

[4] Boyd PW, Watson AJ, Law CS, et al. (October 2000). "A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization". Nature. 407 (6805): 695–702. Bibcode:2000Natur.407..695B. doi:10.1038/35037500. PMID 11048709. S2CID 4368261.

[earn-5] Earnshaw, A.; Greenwood, N. N. (1997). Chemistry of the elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4.

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