Bromoantimonates

Bromoantimonates are compounds containing anions composed of bromide (Br−) and antimony (Sb). They can be considered as double bromides, metallohalides or halometallates. They are in the category of halopnictates. Related compounds include the bromobismuthates, iodoantimonates, bromophosphates, and bromoarsenates.

Some examples of bromoantimonates
Bromoantimonates can have antimony in one of two oxidation states, either +3 or +5. These are designated by bromoantimonate(III) or bromoantimonate(V). Although antimony tribromide is known, SbBr5 on its own does not exist, despite the existence of SbBr6−. Compounds containing both Sb(III) and Sb(V) are mixed valence compounds. The antimony can be linked into chains, (which are called one-dimensional as it is like a line), or in pairs as Sb2 or singly as Sb (which are called zero-dimensional, as the anions are just point-like).

Tetrabromoantimonate(III) (SbBr4−) has single antimony atoms in each anion.

SbBr52− contains infinite chains of SbBr6 octahedra linked by corners.

Hexabromoantimonate(III) SbBr63− also exists, forming zero-dimensional compounds.

Nonabromodiantimonate(III) Sb2Br93− has a pair of octahedra of SbBr6 sharing a face. The angles formed by shared bromine at the antimony are reduced from the theoretical 90° to about 80°.

Hexabromoantimonate(V) SbBr6− exists in equilibrium with SbBr4− and Br2 but real +5 oxidation state bromoantimonates exist even if SbBr5 is unstable and does not exist.

Intersecting classes are nonahalogenodiantimon(III)ates. Hexahaloantimonate(III) ions (SbX63−) can include chlorine or iodine as well as bromine.

Related classes include halogeno-oxydiantimon(III)ates, and pentabromothio‐diantimonates. Bromo-phenylantimonates(V) (C6H5SbBr5) have a phenyl group substituting for one bromine atom. Chlorobromoantimonates(V) have some chlorine instead of bromine.

The antimonide bromides are distinct as they do not have a bond between antimony and bromine. Tetrabromostibonium(V) contains cations (SbBr4+) rather than anions.

Preparation
Bromoantimonates can be made using a water solution of antimony tribromide, hydrobromic acid, and an amine. Alternately antimony trioxide can be used. Variants can be made without using water, and heating the antimony tribromide with an amine hydrobromide.

Some properties
Most of the bromoantimonate salts are yellow or orange.

For SbBr4- there are absorption bands at 224, 300, 245 and 216 nm. SbBr6(3-) has absorption bands centred on 359, 311, and 266 nm.

Many alkylammonium salts are known. These can have different solid phases at different temperatures, as the alkyl group varies in disorder. Those with longer alkyl chains have a greater number of solid state phases.

Methyl ammonium compounds are ferroelectric.

The infrared spectrum includes wavenumbers (ν/c) of 110 to 130 cm−1 for the bridging bromine in Sb2Br9 and 135 to 220 cm−1 for non-bridging bromine to antimony stretching.

Bond lengths in SbBr6− are 2.65 and 2.56 Å. In SbBr63− the lengths are greater, from 2.787 to 2.713 Å.

Use
Bromoantimonates are researched for use in photovoltaics, X-ray detectors, and LEDs.