Iron(III) chloride

All forms of ferric chloride are paramagnetic, owing to the presence of unpaired electrons residing in 3d orbitals. Although Fe(III) chloride can be octahedral or tetrahedral (or both, see structure section), all of these forms have five unpaired electrons, one per d-orbital. The high spin d5 electronic configuration requires that d-d electronic transitions are spin forbidden, in addition to violating the Laporte rule. This double forbidden-ness results in its solutions being only pale colored. Or, stated more technically, the optical transitions are non-intense. Aqueous ferric sulfate and ferric nitrate, which contain , are nearly colorless, whereas the chloride solutions are yellow. Thus, the chloride ligands significantly influence the optical properties of the iron center. ==Structure==

Iron(III) chloride can exist as an anhydrous material and a series of hydrates, which results in distinct structures. Anhydrous The anhydrous compound is a hygroscopic crystalline solid with a melting point of 307.6 °C. The colour depends on the viewing angle: by reflected light, the crystals appear dark green, but by transmitted light, they appear purple-red. Anhydrous iron(III) chloride has the bismuth(III) iodide| structure, with octahedral Fe(III) centres interconnected by two-coordinate chloride ligands. Hydrates Ferric chloride form hydrates upon exposure to water, reflecting its Lewis acidity. All hydrates exhibit deliquescence, meaning that they become liquid by absorbing moisture from the air. Hydration invariably gives derivatives of aquo complexes with the formula . This cation can adopt either trans or cis stereochemistry, reflecting the relative location of the chloride ligands on the octahedral Fe center. Four hydrates have been characterized by X-ray crystallography: the dihydrate , the disesquihydrate , the trisesquihydrate , and finally the hexahydrate . These species differ with respect to the stereochemistry of the octahedral iron cation, the identity of the anions, and the presence or absence of water of crystallization. Solution Like the solid hydrates, aqueous solutions of ferric chloride also consist of the octahedral of unspecified stereochemistry. The polymeric species formed by the hydrolysis of ferric chlorides are key to the use of ferric chloride for water treatment. In contrast to the complicated behavior of its aqueous solutions, solutions of iron(III) chloride in diethyl ether and tetrahydrofuran are well-behaved. Both ethers form 1:2 adducts of the general formula FeCl3(ether)2. In these complexes, the iron is pentacoordinate. ==Preparation==

Several hundred tons of anhydrous iron(III) chloride are produced annually. The principal method, called direct chlorination, uses scrap iron as a precursor: Aqueous solutions of iron(III) chloride are also produced industrially from a number of iron precursors, including iron oxides: : In complementary route, iron metal can be oxidized by hydrochloric acid followed by chlorination: or trimethylsilyl chloride: : : ==Reactions==

Being high spin d5 electronic configuration iron(III) chlorides are labile, meaning that its Cl- and H2O ligands exchange rapidly with free chloride and water. : Alkali metal alkoxides react to give the iron(III) alkoxide complexes. These products have more complicated structures than anhydrous iron(III) chloride. In the solid phase a variety of multinuclear complexes have been described for the nominal stoichiometric reaction between and sodium ethoxide: : Iron(III) chloride forms a 1:2 adduct with Lewis bases such as triphenylphosphine oxide; e.g., . The related 1:2 complex , has been crystallized from ether solution. Iron(III) chloride also reacts with tetraethylammonium chloride to give the yellow salt of the tetrachloroferrate ion (). Similarly, combining FeCl3 with NaCl and KCl gives and , respectively. In addition to these simple stoichiometric reactions, the Lewis acidity of ferric chloride enables its use in a variety of acid-catalyzed reactions as described below in the section on organic chemistry. : iron(III) chloride releases chlorine gas when heated above 160 °C, generating ferrous chloride: The affinity of iron(III) for phenols is exploited in the Trinder spot test. Aqueous iron(III) chloride serves as a one-electron oxidant illustrated by its reaction with copper(I) chloride to give copper(II) chloride and iron(II) chloride. : This fundamental reaction is relevant to the use of ferric chloride solutions in etching copper. Organometallic chemistry The interaction of anhydrous iron(III) chloride with organolithium and organomagnesium compounds has been examined often. These studies are enabled because of the solubility of FeCl3 in ethereal solvents, which avoids the possibility of hydrolysis of the nucleophilic alkylating agents. Such studies may be relevant to the mechanism of FeCl3-catalyzed cross-coupling reactions. The isolation of organoiron(III) intermediates requires low-temperature reactions, lest the [FeR4]− intermediates degrade. Using methylmagnesium bromide as the alkylation agent, salts of Fe(CH3)4]− have been isolated. Illustrating the sensitivity of these reactions, methyl lithium reacts with iron(III) chloride to give lithium tetrachloroferrate(II) : : : To a significant extent, iron(III) acetylacetonate and related beta-diketonate complexes are more widely used than FeCl3 as ether-soluble sources of ferric ion. :{{chem2|3 C5H5MgBr + FeCl3 -> Fe(C5H5)2 + 1/n (C5H5)_{n} + 3 MgBrCl}} This conversion, although not of practical value, was important in the history of organometallic chemistry where ferrocene is emblematic of the field. ==Uses==

Water treatment The largest applications of iron(III) chloride are sewage treatment and drinking water production. By forming highly dispersed networks of Fe-O-Fe containing materials, ferric chlorides serve as coagulant and flocculants. In this application, an aqueous solution of is treated with base to form a floc of iron(III) hydroxide (), also formulated as FeO(OH) (ferrihydrite). This floc facilitates the separation of suspended materials, clarifying the water. Iron(III) chloride is also used to remove soluble phosphate from wastewater. Iron(III) phosphate is insoluble and thus precipitates as a solid. One potential advantage of its use in water treatment, is that the ferric ion oxidizes (deodorizes) hydrogen sulfide. Etching and metal cleaning It is also used as a leaching agent in chloride hydrometallurgy, for example in the production of Si from FeSi (Silgrain process by Elkem). In another commercial application, a solution of iron(III) chloride is useful for etching copper according to the following equation: : The soluble copper(II) chloride is rinsed away, leaving a copper pattern. This chemistry is used in the production of printed circuit boards (PCB). Iron(III) chloride is used in many other hobbies involving metallic objects. Organic chemistry In industry, iron(III) chloride is used as a catalyst for the reaction of ethylene with chlorine, forming ethylene dichloride (1,2-dichloroethane): : Ethylene dichloride is a commodity chemical, which is mainly used for the industrial production of vinyl chloride, the monomer for making PVC. Illustrating it use as a Lewis acid, iron(III) chloride catalyses electrophilic aromatic substitution and chlorinations. In this role, its function is similar to that of aluminium chloride. In some cases, mixtures of the two are used. Organic synthesis research Although iron(III) chlorides are seldom used in practical organic synthesis, they have received considerable attention as reagents because they are inexpensive, earth abundant, and relatively nontoxic. Many experiments probe both its redox activity and its Lewis acidity. 3-Alkylthiophenes are polymerized to polythiophenes upon treatment with ferric chloride. Iron(III) chloride has been shown to promote C-C coupling reaction. Several reagents have been developed based on supported iron(III) chloride. On silica gel, the anhydrous salt has been applied to certain dehydration and pinacol-type rearrangement reactions. A similar reagent but moistened induces hydrolysis or epimerization reactions. On alumina, ferric chloride has been shown to accelerate ene reactions. When pretreated with sodium hydride, iron(III) chloride gives a hydride reducing agent that convert alkenes and ketones into alkanes and alcohols, respectively. Histology Iron(III) chloride is a component of useful stains, such as Carnoy's solution, a histological fixative with many applications. Also, it is used to prepare Verhoeff's stain. ==Natural occurrence==

Like many metal halides, naturally occurs as a trace mineral. The rare mineral molysite is usually associated with volcanoes and fumaroles. -based aerosol are produced by a reaction between iron-rich dust and hydrochloric acid from sea salt. This iron salt aerosol causes about 1–5% of naturally occurring oxidization of methane and is thought to have a range of cooling effects; thus, it has been proposed as a catalyst for Atmospheric Methane Removal. The clouds of Venus are hypothesized to contain approximately 1% dissolved in sulfuric acid. == Safety ==

Iron(III) chlorides are widely used in the treatment of drinking water, ==Notes==