Oxohalide

 ( Oxohalides ) 

In chemistry, oxohalides or oxyhalides are a group of chemical compounds with the chemical formula , where X is a halogen, and A is an element different than O and X. Oxohalides are numerous. Molecular oxohalides are , whereas nonmolecular oxohalides are . Some oxohalides of particular practical significance are phosgene (COCl₂), thionyl chloride (SOCl₂), and sulfuryl fluoride (SO₂F₂).

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Synthesis Oxohalides can be seen as compounds intermediate between and . There are three general methods of synthesis:Synthesis of individual compounds can be found in Housecroft & Sharpe and Greenwood & Earnshaw in sections relating to the specific element, A

• Partial oxidation of a halide:
• :
• In this example, the oxidation state increases by two and the electrical charge is unchanged.
• Partial halogenation of an oxide:
• :
• Oxide replacement:
• :

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 .

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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 of the central atom decreases by one. For example, both phosphorus oxychloride () and phosphorus pentachloride, () are neutral covalent compounds of phosphorus in the +5 oxidation state.

Oxohalides of elements in high oxidation states can be strong , with oxidizing power similar to the corresponding oxide or halide. Most oxohalides are easily hydrolysis. 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 reactionGreenwood & Earnshaw, p. 1023

Many oxohalides can act as . 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 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.Greenwood & Earnshaw, p. 996.

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 and this is backed up by measurements of M–O bond lengths. It implies that the elements A and O are chemical bond 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.Shriver & Atkins, Figure 13.8, p. 447

[[File:Boron-teflate-3D-balls.png|thumb|170px|Boron teflate.

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Main group elements

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Carbon group Carbon Carbon oxohalide , X = F, Carbonyl bromide, and the very toxic phosgene (X = Cl), 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.Shriver & Atkins, p. 358 For example:

Silicon tetrafluoride reacts with water to yield poorly-characterized oxyfluoride polymers, but slow and careful reaction at liquid nitrogen yields the oxyfluoride hexafluorodisiloxane as well.

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Pnictogens Nitrogen forms two series of oxohalides with nitrogen in oxidation states 3, NOX, X = F, Cl, Nitrosyl bromide and 5, , X = nitryl fluoride, Cl. They are made by halogenation of nitrogen oxides. Note that is isoelectronic with the nitrate ion, . Only oxohalides of phosphorus(V) are known.Housecroft & Sharpe, pp. 329–330 Examples are phosphoryl chloride and pyrophosphoryl chloride .

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Chalcogens Sulfur forms oxohalidesHousecroft & Sharpe, pp. 365–367 in oxidation state +4, such as thionyl chloride, and oxidation state +6, such as sulfuryl fluoride (), sulfuryl chloride (), and thionyl tetrafluoride (). 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

Selenium and tellurium 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 Teflic acid, is a large and rather stable anion, useful for forming stable salts with large cations.

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Halogens

The halogens form various oxofluorides with formulae (e.g. chloryl fluoride), (e.g. perchloryl fluoride) and with X = Cl, Br and I. and are also known.Housecroft & Sharpe, p. 395

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Noble gases

Xenon forms xenon oxytetrafluoride (), xenon dioxydifluoride () and xenon oxydifluoride ().

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Transition metals and actinides

.

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A selection of known oxohalides of is shown below, and more detailed lists are available in the literature.Greenwood & Earnshaw, Chapters 22–25, section halides and oxohalides X indicates various halides, most often F and Cl.

VOCl, VOBr,Greenwood & Earnshaw p. 993. FeOCl

, , ,

, , , , , , , ,

, , , , , , , , , , , , , ,

, , , , ,

,

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 for compounds with d⁰ configuration are predicted by VSEPR theory. Thus, is tetrahedral, is trigonal bipyramidal, is and is octahedral.Housectroft & Sharpe, Chapters 21 and 22 illustrate many structures, including M-O and M-Cl bond lengths. The d¹ complex is square pyramidal.

The compounds and (M = W, Ru, Os) have two groups joined by a bridging oxygen atom.. The structure is illustrated in Housectroft & Sharpe, Figure 22.5. Each metal has an octahedral environment. The unusual linear structure can be rationalized in terms of molecular orbital theory, indicating the presence of d_π — p_π bonding between the metal and oxygen atoms.Housectroft & Sharpe, Figure 22.15. Oxygen bridges are present in more complex configurations like (M = Ti, Zr, Hf, Mo or W; cp = cyclopentadienyl, ) or .

In the actinide series, uranyl compounds such as uranyl chloride () and are well known and contain the linear moiety. Similar species exist for neptunium and plutonium. The species uranyl fluoride is a complicating contaminant in samples uranium hexafluoride.

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Minerals and ionic compounds

Mineral oxohalides are rare, but a few are known, such as bismuth oxochloride (BiOCl, bismoclite). Another mineral is terlinguaite , formed by the weathering of mercury-containing minerals. Mendipite, , formed from an original deposit of lead sulfide in a number of stages, is another example of a secondary oxohalide mineral.

The elements iron, antimony, bismuth and lanthanum 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

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See also

• Transition metal oxo complex

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Bibliography

• Housecroft, C. E. and Sharpe, A. G. Inorganic Chemistry, 2nd ed., Pearson Prentice-Hall 2005.
• Shriver, D. F. and Atkins, P. W. Inorganic Chemistry, 3rd edn. Oxford University Press, 1999.
• A. F. Wells (1962). 9780198551256, Clarendon Press. ISBN 9780198551256
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Categories: Oxohalides

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