Molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula AOmXn, X = F, Cl, Br, I. The element A may be a main group element, a transition element or an actinide. The term oxohalide, or oxyhalide, may also refer to minerals and other crystalline substances with the same overall chemical formula, but having an ionic structure.
Synthesis and properties
- Partial oxidation of a halide: 2PCl3 + O2 → 2POCl3. In this example, the oxidation state increases by two and the electrical charge is unchanged.
- Partial halogenation of an oxide: 2V2O5 + 6Cl2 + 3C → 4VOCl3 + 3CO2
- Oxide replacement: [CrO4]2− + 2Cl− + 4H+ → CrO2Cl2 + 4H2O
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 POFCl2 and CrO2FCl.
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 of the central atom decreases by one. For example, both phosphorus oxychloride, POCl3 and phosphorus pentachloride, PCl5 are neutral covalent compounds of phosphorus in the +5 oxidation state. 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.
- [Cr2O7]2− + 4Cl− + 6H+ → 2CrO2Cl2 + 3H2O
Oxohalides of elements in high oxidation states are strong oxidizing agents, 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 contribution to the Gibbs free energy change for the reaction
Many oxohalides can act as Lewis acids. This is particularly so with oxohalides of coordination number 3 or 4 which, in accepting one or more electron pairs from a Lewis base, become 5- or 6- coordinate. Oxohalide anions such as [VOCl4]2− can be seen as acid-base complexes of the oxohalide (VOCl2) with more halide ions acting as Lewis bases. Another example is VOCl2 which forms the trigonal bipyramidal complex VOCl2(N(CH3)3)2 with the base trimethylamine.
The vibrational spectra of many oxohalides have been assigned in detail. They give useful information on relative bond strengths. For example, in CrO2F2, 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 bonds 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.
Main group elements
- Carbon group: Carbon forms oxohalides COX, X=F, Br, and the very toxic phosgene, which is produced industrially by a carbon-catalyzed reaction of carbon monoxide with chlorine. It is a useful reagent in organic chemistry for the formation of carbonyl compounds. For example,
- COCl2 + 2 ROH → CO(OR)2 + 2HCl
- Pnictogens: Nitrogen forms two series of oxohalides with nitrogen in oxidation states 3, NOX, X=F, Cl, Br and 5, NO2X, X=F, Cl. They are made by halogenation of nitrogen oxides. Note that NO2F is isoelectronic with the nitrate ion, NO3−. Only oxohalides of phosphorus(V) are known.
- Chalcogens: Sulfur forms oxohalides in oxidation state +4, such as thionyl chloride, SOCl2 and oxidation state +6, such as sulfuryl fluoride, SO2F2 and thionyl tetrafluoride, SOF4. All are easily hydrolyzed. Indeed, thionyl chloride can be used as a dehydration agent as the water molecules are converted into gaseous products, leaving the anhydrous solid behind.
- MgCl2·6H2O + 6SOCl2 → MgCl2 + 6SO2 + 12 HCl
Selenium and tellurium form similar compounds and also the oxo-bridged species F5AOAF5 (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 [TeOF5]−, 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 XO2F, XO3F and XOF3 with X = Cl, Br and I. IO2F3 and IOF5 are also known.
- Noble gases: Xenon oxytetrafluoride, XeOF4
Transition metals and actinides
Oxidation state 4: [TiOCl4]2−, Cl3TiOTiCl3, VOCl2, [VOCl4]2−
Oxidation state 7: ReOF5, ReO2F3, ReO3Cl, OsOF5
Oxidation state 8: OsO2F4, OsO3F2
High oxidation states of the metal are dictated by the fact that oxygen is a strong oxidizing agent, as is fluorine. Bromine and iodine are relatively weak oxidizing agents, so fewer oxobromides and oxoiodides are known. Structures are easily predicted by VSEPR theory. Thus, CrO2Cl2 is tetrahedral, OsO3F2 is trigonal bipyramidal, XeOF4 is square pyramidal and OsOF5 is octahedral. An interesting case is ReOCl4 which is square pyramidal, with a single electron in the 6th position.
The compounds [Ta2OX10]2− and [M2OCl10]4− (M = W, Ru, Os) have two MX5 groups joined by a bridging oxygen atom. Each metal has an octahedral environment. The unusual linear M—O—M structure can be rationalized in terms of molecular orbital theory, indicating the presence of dπ — pπ bonding between the metal and oxygen atoms. Oxygen bridges are present in more complex configurations like M(cp)2(OTeF5)2 (M = Ti, Zr, Hf, Mo or W; cp = η5−C5H5) or [AgOTeF5−(C6H5CH3)2]2.
Minerals and ionic compounds
Bismuth oxochloride (BiOCl, bismoclite) is a rare example of a mineral oxohalide. The crystal structure has a tetragonal 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 Hg2OCl, formed by the weathering of mercury-containing minerals. Mendipite, Pb3O2Cl2, formed from an original deposit of lead sulfide in a number of stages is another example of a secondary oxohalide mineral.
- Synthesis of individual compounds can be found in Housecroft&Sharpe and Greenwood&Earnshaw in sections relating to the specific element, A
- Sisler, H. H. "Chromyl Chloride" Inorganic Synthesis McGraw-Hill: New York, 1946; Vol. 2, pp. 205–207.
- Greenwood & Earnshaw, p. 1023
- Greenwood & Earnshaw, p. 996.
- K. Nakamoto Infrared and Raman spectra of inorganic and coordination compounds, 5th. edition, Part A, Wiley, 1997 ISBN 0-471-19406-9, Tables II-4c, II-6g, II-6h, II-7b, II-8c
- Shriver & Atkins, Figure 13.8, p. 447
- Shriver & Atkins, p. 358
- Housecroft & Sharpe, pp. 329–330
- Housecroft & Sharpe, pp. 365–367
- Shriver & Atkins, p. 397
- Oberhammer, Heinz; Seppelt, Konrad (1978). "Molecular Structure of F5SOSF5, F5SeOSeF5, and F5TeOTeF5: d-Orbital Participation in Bonds between Main Group Elements". Angewandte Chemie International Edition 17 (1): 69–70. doi:10.1002/anie.197800691.
- Housecroft & Sharpe, p. 395
- Fourati, Mohieddine; Chaabouni, Moncef; Belin, Claude Henri; Charbonnel, Monique; Pascal, Jean Louis; Potier, Jacqueline (1986). "A strongly chelating bidentate perchlorate. New synthesis route and crystal structure determination of titanium(4+) perchlorate". Inorg. Chem. 25 (9): 1386–1390. doi:10.1021/ic00229a019.
- Greenwood & Earnshaw, Chapters 22–25, section halides and oxohalides
- Greenwood & Earnshaw p. 993.
- Strauss, Steven H.; Noirot, Mark D.; Anderson, Oren P. (1985). "Preparation and characterization of silver(I) teflate complexes: bridging OTeF5 groups in the solid state and in solution". Inorg. Chem. 24 (25): 4307–4311. doi:10.1021/ic00219a022.
- Housectroft & Sharpe, Chapters 21 and 22 illustrate many structures, including M-O and M-Cl bond lengths.
- Dewan, John. C.; Edwards, Anthony J.; Calves, Jean Y.; Guerchais, Jacques E. (1997). "Fluoride crystal structures. Part 28. Bis(tetraethylammonium)µ-oxo-bis[pentafluorotantalate(V)]". J. Chem. Soc., Dalton Trans. (10): 978–980. doi:10.1039/DT9770000978.. The structure is illustrated in Housectroft & Sharpe, Figure 22.5.
- Housectroft & Sharpe, Figure 22.15.
- Crossman, Martin C.; Hope, Eric G.; Saunders, Graham C. (1996). "Cyclopentadienyl metal teflate (OTeF5) complexes". J. Chem. Soc., Dalton Trans. (4): 509–511. doi:10.1039/DT9960000509.
- Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C. (ed.). "Bismoclite" (PDF). Handbook of Mineralogy. III (Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0-9622097-2-4. Retrieved December 5, 2011.
- Hillebrand, W. F.; W. T. Schaller (1907). "Art. XXVI. "The Mercury Minerals from Terlingua, Texas: Kleinite, Terlinguaite, Eglestonite, Montroydite, Calomel, Mercury"". The American Journal of Science (139): 259–274. Retrieved 2009-05-21.
- Wells, pp. 390–392
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- Housecroft, C. E. and Sharpe,A. G. “Inorganic Chemistry”, 2nd edn, Pearson Prentice-Hall 2005. ISBN 0-582-31080-6
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- Wells, A.F (1962). Structural Inorganic Chemistry (3rd. ed.). Oxford: Clarendon Press. pp. 384–392. ISBN 0-19-855125-8..