page_title: Oxohalide
section_title: Properties
hierarchical_section_title: Oxohalide / Properties
caption_attribution_description: Ball-and-stick model of the boron teflate molecule, a compound derived from teflic acid. Colour&#160;code: &#160;&#160;Oxygen,&#160;O:&#160;red &#160;&#160;Boron,&#160;B:&#160;pink &#160; Yellow-green: Fluorine, F &#160;&#160;Tellurium,&#160;Te:&#160;brown
context_page_description: Molecular oxohalides 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.
context_section_description: 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
  The chromyl chloride 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 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.
caption: Boron teflate. Colours: pink – B, red – O, brown – Te, green – F.
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