Oxohalide

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

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 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: CrO2−4 + 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 POFCl2 and CrO2FCl.

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 (POCl3) and phosphorus pentachloride, (PCl5) are neutral covalent compounds of phosphorus in the +5 oxidation state. Oxohalides of elements in high oxidation states can be 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. Oxohalides of elements in high oxidation states are intensely coloured owing to ligand to metal charge transfer (LMCT) transitions.

Main group elements

Carbon group Carbon forms oxohalides COX2, X = F, Br, 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. For example:

COCl2 + 2 ROH → CO(OR)2 + 2 HCl Silicon tetrafluoride reacts with water to yield poorly-characterized oxyfluoride polymers, but slow and careful reaction at -196 °C yields the oxyfluoride hexafluorodisiloxane as well.

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, NO−3. Only oxohalides of phosphorus(V) are known. Examples are phosphoryl chloride POCl3 and pyrophosphoryl chloride P2O3Cl4.

Chalcogens Sulfur forms oxohalides in oxidation state +4, such as thionyl chloride, SOCl2 and oxidation state +6, such as sulfuryl fluoride (SO2F2), sulfuryl chloride (SO2Cl2), 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 behind the anhydrous solid chloride.

MgCl2·6H2O + 6 SOCl2 → MgCl2 + 6 SO2 + 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 F5TeO−, 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 formulas XO2F (e.g. chloryl fluoride), XO3F (e.g. perchloryl fluoride) and XOF3 with X = Cl, Br and I. IO2F3 and IOF5 are also known.

Noble gases Xenon forms xenon oxytetrafluoride (XeOF4), xenon dioxydifluoride (XeO2F2) and xenon oxydifluoride (XeOF2).

Transition metals and actinides

A selection of known oxohalides of transition metals is shown below, and more detailed lists are available in the literature. X indicates various halides, most often F and Cl.

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