In chemistry, molecular oxohalides (oxyhalides) are a group of

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

s in which both

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

and halogen atoms are attached to another

chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

A in a single

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

. They have the general formula , where X = fluorine (F),

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

(Cl),

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...

(Br), and/or iodine (I). The element A may be a main group element, a

transition element In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...

or an

actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The info ...

. The term ''oxohalide'', or ''oxyhalide'', may also refer to

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s and other

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

substances with the same overall chemical formula, but having an ionic structure.

Synthesis

Oxohalides can be seen as compounds intermediate between oxides and halides. There are three general methods of synthesis: *Partial oxidation of a halide: *:2 PCl3 + O2 -> 2 POCl3 **In this example, the

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

increases by two and the electrical charge is unchanged. *Partial halogenation of an oxide: *:2 V2O5 + 6 Cl2 + 3 C -> 4 VOCl3 + 3 CO2 *Oxide replacement: *: rO42- + 2 Cl- + 4 H+ -> CrO2Cl2 + 4 H2O In addition, various oxohalides can be made by halogen exchange reactions and this reaction can also lead to the formation of mixed oxohalides such as and .

Properties

In relation to the oxide or halide, for a given oxidation state of an element A, if two halogen atoms replace one oxygen atom, or ''vice versa'', the overall charge on the molecule is unchanged and the

coordination number In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central io ...

of the central atom decreases by one. For example, both

phosphorus oxychloride Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula . It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from pho ...

() and

phosphorus pentachloride Phosphorus pentachloride is the chemical compound with the formula PCl5. It is one of the most important phosphorus chlorides, others being PCl3 and POCl3. PCl5 finds use as a chlorinating reagent. It is a colourless, water-sensitive and moist ...

, () are neutral

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

compounds of

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

in the +5

. If an oxygen atom is simply replaced by a halogen atom the charge increases by +1, but the coordination number is unchanged. This is illustrated by the reaction of a mixture of a chromate or dichromate salt and potassium chloride with concentrated sulfuric acid. : r2O72- + 4 Cl- + 6 H+ -> 2 CrO2Cl2 + 3 H2O The

chromyl chloride Chromyl chloride is the inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal complexes. Preparation Chromyl chloride can be prepared by th ...

produced has no electrical charge and is a volatile covalent molecule that can be distilled out of the reaction mixture. Oxohalides of elements in high oxidation states are strong

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...

s, with oxidizing power similar to the corresponding oxide or halide. Most oxohalides are easily hydrolyzed. For example, chromyl chloride is hydrolyzed to chromate in the reverse of the synthetic reaction, above. The driving force for this reaction is the formation of A-O bonds which are stronger than A-Cl bonds. This gives a favourable

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...

contribution to the

Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and ...

change for the reaction Many oxohalides can act as Lewis acids. This is particularly so with oxohalides of

3 or 4 which, in accepting one or more electron pairs from a Lewis base, become 5- or 6- coordinate. Oxohalide anions such as can be seen as acid-base complexes of the oxohalide () with more halide ions acting as Lewis bases. Another example is which forms the trigonal bipyramidal complex with the base

trimethylamine Trimethylamine (TMA) is an organic compound with the formula N(CH3)3. It is a colorless, hygroscopic, and flammable tertiary amine. It is a gas at room temperature but is usually sold as a 40% solution in water. (It is also sold in pressurized ...

. The vibrational spectra of many oxohalides have been assigned in detail. They give useful information on relative bond strengths. For example, in , the Cr–O stretching vibrations are at 1006 cm⁻¹ and 1016 cm⁻¹ and the Cr–F stretching vibrations are at 727 cm⁻¹ and 789 cm⁻¹. The difference is much too large to be due to the different masses of O and F atoms. Rather, it shows that the Cr–O bond is much stronger than the Cr–F bond. M–O bonds are generally considered to be

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

s and this is backed up by measurements of M–O bond lengths. It implies that the elements A and O are chemically bound together by a σ bond and a π bond.K. Nakamoto ''Infrared and Raman spectra of inorganic and coordination compounds'', 5th. edition, Part A, Wiley, 1997 , Tables II-4c, II-6g, II-6h, II-7b, II-8c Oxohalides of elements in high oxidation states are intensely coloured owing to ligand to metal charge transfer (LMCT) transitions. Boron-teflate-3D-balls

Main group elements

Carbon group The carbon group is a periodic table group consisting of carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and flerovium (Fl). It lies within the p-block. In modern IUPAC notation, it is called group 14. In the field of sem ...

: Carbon forms oxohalides , X = F, Br, and the very toxic phosgene (X = Cl), which is produced industrially by a carbon-catalyzed reaction of

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

with

. It is a useful reagent in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...

for the formation of

carbonyl compounds In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...

. For example, *:COCl2 + 2 ROH -> CO(OR)2 + 2 HCl *

Pnictogen A pnictogen ( or ; from grc, πνῑ́γω "to choke" and -gen, "generator") is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the el ...

Nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

forms two series of oxohalides with nitrogen in oxidation states 3, NOX, X = F, Cl, Br and 5, , X = F, Cl. They are made by halogenation of nitrogen oxides. Note that is

isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...

with the nitrate ion, . Only oxohalides of

(V) are known. *

Chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioac ...

s: Sulfur forms oxohalides in oxidation state +4, such as

thionyl chloride Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...

, and oxidation state +6, such as

sulfuryl fluoride Sulfuryl fluoride (also spelled ''sulphuryl fluoride'') is an inorganic compound with the formula SO2F2. It is an easily condensed gas and has properties more similar to sulfur hexafluoride than sulfuryl chloride, being resistant to hydrolysis eve ...

(),

sulfuryl chloride Sulfuryl chloride is an inorganic compound with the formula SO2Cl2. At room temperature, it is a colorless liquid with a pungent odor. Sulfuryl chloride is not found in nature, as can be inferred from its rapid hydrolysis. Sulfuryl chloride is ...

(), and

thionyl tetrafluoride Thionyl tetrafluoride is an inorganic compound gas with the formula S O F4. It is also known as sulfur tetrafluoride oxide. The shape of the molecule is a distorted trigonal bipyramid, with the oxygen found on the equator. The atoms on the e ...

(). All are easily hydrolyzed. Indeed, thionyl chloride can be used as a dehydration agent as the water molecules are converted into gaseous products, leaving behind the anhydrous solid chloride.Shriver & Atkins, p. 397 *:MgCl2*6H2O + 6 SOCl2 -> MgCl2 + 6 SO2 + 12 HCl

Selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

and

tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...

form similar compounds and also the oxo-bridged species (A = S, Se, Te). They are non-linear with the A-O-A angle of 142.5, 142.4 and 145.5° for S, Se and Te, respectively. The tellurium anion , known as teflate, is a large and rather stable anion, useful for forming stable salts with large cations. * Halogens: The halogens form various oxofluorides with formulae (

chloryl fluoride Chloryl fluoride is the chemical compound with the formula ClO2F. It is commonly encountered as side-product in reactions of chlorine fluorides with oxygen sources. It is the acyl fluoride of chloric acid. Preparation ClO2F was first reported by ...

), (

perchloryl fluoride Perchloryl fluoride is a reactive gas with the chemical formula . It has a characteristic sweet odor that resembles gasoline and kerosene. It is toxic and is a powerful oxidizing and fluorinating agent. It is the acid fluoride of perchloric acid. ...

) and with X = Cl, Br and I. and are also known. *

Noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...

es:

xenon oxytetrafluoride Xenon oxytetrafluoride () is an inorganic chemical compound. It is a colorless stable liquid with a melting point of that can be synthesized by partial hydrolysis of , or the reaction of with silica or : : + → + + A high-yield synthesi ...

()

Transition metals and actinides

A selection of known oxohalides of

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...

s is shown below, and more detailed lists are available in the literature. X indicates various halides, most often F and Cl. {, class="wikitable" ! Oxidation state !! oxohalides , - , 3, , VOCl, VOBr, FeOCl , - , 4, , , , , , - , 5, , , {{chem2, VO2X, {{chem2, rOF4, {{chem2, rOF52-), {{chem2, MnOCl3, {{chem2, TcOCl3, {{chem2, VOF3, {{chem2, VOCl3, {{chem2, NbOCl3 , - , 6, , {{chem2, CrO2Cl2, {{chem2, rO3Cl, {{chem2, ReOX4, {{chem2, ReO2F2, {{chem2, OsOF4, {{chem2, CrO2F2, {{chem2, MoOCl4, {{chem2, MoO2Cl2, {{chem2, WO2Cl2, {{chem2, WOCl4 , - , 7, , {{chem2, MnO3Cl, {{chem2, ReOF5, {{chem2, ReO2F3, {{chem2, ReO3Cl, {{chem2, OsOF5 , - , 8, , {{chem2, OsO2F4, {{chem2, OsO3F2 Ta2OCl10 2-

High oxidation states of the metal are dictated by the fact that

is a strong

, as is fluorine.

Bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...

and iodine are relatively weak oxidizing agents, so fewer oxobromides and oxoiodides are known. Structures for compounds with d⁰ configuration are predicted by

VSEPR theory Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm the ...

. Thus, {{chem2, CrO2Cl2 is

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...

, {{chem2, OsO3F2 is trigonal bipyramidal, {{chem2, XeOF4 is square pyramidal and {{chem2, OsOF5 is

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

. The d¹ complex {{chem2, ReOCl4 is square pyramidal. The compounds {{chem2, a2OX102-) and {{chem2, 2OCl104-) (M = W, Ru, Os) have two {{chem2, MX5 groups joined by a bridging oxygen atom. Each metal has an octahedral environment. The unusual linear {{chem2, M\sO\sM structure can be rationalized in terms of

molecular orbital In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of findin ...

theory, indicating the presence of d_π — p_π bonding between the metal and oxygen atoms. Oxygen bridges are present in more complex configurations like {{chem2, M(cp)2(OTeF5)2 (M = Ti, Zr, Hf, Mo or W; cp =

cyclopentadienyl complex A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (''η''5-) bonding mode. The metal–cyclopentadien ...

, {{chem2, \h{5}C5H5) or {{chem2, gOTeF5\-(C6H5CH3)2. In the

series,

uranyl The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula . It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may ...

compounds such as

uranyl chloride Uranyl chloride refers to inorganic compounds with the formula UO2Cl2(H2O)n where n = 0, 1, or 3. These are yellow-colored solids. Synthesis and structures The hydrates are obtained by dissolving uranyl sulfate or uranyl acetate in hydrochloric ...

({{chem2, UO2Cl2) and {{chem2, O2Cl42-) are well known and contain the linear {{chem2, UO2 moiety. Similar species exist for

neptunium Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it bein ...

and

plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...

Minerals and ionic compounds

Bismuth oxochloride (BiOCl, bismoclite) is a rare example of a mineral oxohalide. The

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...

has a

tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...

symmetry and can be thought of as consisting of layers of Cl⁻, Bi³⁺ and O²⁻ ions, in the order Cl-Bi-O-Bi-Cl-Cl-Bi-O-Bi-Cl. This layered, graphite-like structure results in a relatively low hardness of bismoclite ( Mohs 2–2.5) and most other oxohalide minerals. Those other minerals include

terlinguaite Terlinguaite is the naturally occurring mineral with formula Hg2 Cl O. It is formed by the weathering of other mercury-containing minerals. It was discovered in 1900 in the Terlingua District of Brewster County Brewster County is a county l ...

Hg₂OCl, formed by the weathering of mercury-containing minerals.

Mendipite Mendipite is a rare mineral that was named in 1939 for the locality where it is found, the Mendip Hills in Somerset, England. It is an oxyhalide of lead with formula Pb3Cl2O2. Crystal structure Most references assert that mendipite crystallis ...

, Pb₃O₂Cl₂, formed from an original deposit of

lead sulfide Lead sulfide refers to two compounds containing lead and sulfur: * Lead(II) sulfide, PbS, containing lead in the +2 oxidation state, naturally occurring as the mineral galena *Lead(IV) sulfide Lead(IV) sulfide is a chemical compound with the formu ...

in a number of stages is another example of a secondary oxohalide mineral. The elements

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient t ...

bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...

and

lanthanum Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lant ...

form oxochlorides of general formula MOCl. MOBr and MOI are also known for Sb and Bi. Many of their crystal structures have been determined.Wells, pp. 390–392

References

{{reflist, 30em, refs= {{cite journal, doi=10.1039/DT9960000509 , title=Cyclopentadienyl metal teflate (OTeF₅) complexes, journal=J. Chem. Soc., Dalton Trans., year= 1996, pages= 509–511, last1=Crossman, first1=Martin C., last2=Hope, first2=Eric G., last3=Saunders, first3=Graham C., issue=4 {{cite journal, title=Preparation and characterization of silver(I) teflate complexes: bridging OTeF₅ groups in the solid state and in solution, journal=Inorg. Chem., year= 1985, volume= 24 , issue=25, pages=4307–4311, doi=10.1021/ic00219a022, last1=Strauss, first1=Steven H., last2=Noirot, first2=Mark D., last3=Anderson, first3=Oren P. {{cite journal, title=Molecular Structure of F₅SOSF₅, F₅SeOSeF₅, and F₅TeOTeF₅: d-Orbital Participation in Bonds between Main Group Elements, doi=10.1002/anie.197800691, journal=Angewandte Chemie International Edition , volume =17, issue =1, pages =69–70, year=1978, last1=Oberhammer, first1=Heinz, last2=Seppelt, first2=Konrad {{cite book, editor1=Anthony, John W. , editor2=Bideaux, Richard A. , editor3=Bladh, Kenneth W. , editor4=Nichols, Monte C. , title= Handbook of Mineralogy, publisher= Mineralogical Society of America, location= Chantilly, VA, url=http://rruff.geo.arizona.edu/doclib/hom/bismoclite.pdf , chapter=Bismoclite , access-date=December 5, 2011 , volume=III (Halides, Hydroxides, Oxides), isbn=0-9622097-2-4

Bibliography

{{Commons category, Oxohalides *{{Greenwood&Earnshaw *Housecroft, C. E. and Sharpe, A. G. ''Inorganic Chemistry'', 2nd ed., Pearson Prentice-Hall 2005. {{ISBN, 0-582-31080-6 *Shrivr, D. F. and Atkins, P. W. ''Inorganic Chemistry'', 3rd edn. Oxford University Press, 1999. {{ISBN, 0-19-850330-X *{{cite book , last=Wells , first=A. F. , title=Structural Inorganic Chemistry , edition=3rd , year=1962 , publisher=Clarendon Press , location=Oxford , pages=384–392 , isbn=0-19-855125-8. Oxohalides