Perchloratoborate
# Perchloratoborate

Perchloratoborate is an anion of the form $[B(ClO_4)_4]^−.$ It can form partly stable solid salts with heavy alkali metals. They are more stable than nitratoborate salts. $K[B(ClO_4)_4]$ decomposes at 35 °C, $Rb[B(ClO_4)_4]$ is stable to 50 °C, and $Cs[B(ClO_4)_4]$ can exist up to 80 °C.

Perchloratoborates are analogous to perchloratoaluminates $([Al(ClO_4)_4]^−).$

Another related anion is the chloroperchloratoborate, $Cl_3B(ClO_4).$

Boron perchlorate itself is unstable above −5 °C.

## Decomposition

On thermal decomposition the alkali perchloratoborate salts form an alkali perchlorate, and boron trioxide as a solid residue, and gas containing dichlorine heptoxide, chlorine dioxide, chlorine, and oxygen.

    2 $M[B(ClO_4)_4]$ → 2 $MClO_4$ + $B_2O_3$ + (3 $Cl_2O_7$ or 6 $ClO_2$ + 4+1/2 $O_2$ or 6 $Cl_2$ + 10+1/2 $O_2)$

When the alkali perchloratoborates first start to decompose at the lower temperatures, the reaction is endothermic, and dichlorine heptoxide is formed. However, if caesium perchloratoborate is heated the decomposition becomes exothermic above 90 °C, and at 100 °C it explodes exothermically forming chlorine and oxygen.

## Reactions

When rubidium perchloratoborate is reacted with extra perchloric acid, it forms $RbBO(ClO_4)_2.$

In water, alkali perchloratoborates decompose exothermically to form boric acid, perchloric acid, and the perchlorate.

## Formation

Nitronium perchloratoborate $(NO_2B(ClO_4)_4)can$ be formed by reacting nitronium perchlorate with boron trichloride in solution. Similarly ammonium perchlorate reacts with $BCl_3$ forming ammonium perchloratoborate.

The metal perchloratoborates can also be formed from the metal perchlorate dissolved in anhydrous perchloric acid reacting with boron trichloride. Another way is to react a metal chloridoborate $(MBCl_4)$ with perchloric acid. Chloridoborates can be made from the metal chloride and boron trichloride dissolved in nitrosyl chloride. Extra $Cl_2O_7$ drives the reaction forward.

    $BCl_3$ + $3HClO_4$ → $B(ClO_4)_3$

Also formed is $BCl_2ClO_4$ and $BCl(ClO_4)_2$ which disproportionates above −78 °C to the boron perchlorate and dichloroboron perchlorate.

    $B(ClO_4)_3$ + $ClO^−$ $_4$ → B(ClO $_4)^−$ $_4$

## Properties

Caesium perchloratoborate is hygroscopic. It has a density of 2.5 $g/cm^3.$ It has no colour.

Infrared absorption bands are observed in caesium perchloratoborate at 640 and 1,087 $cm^{−1}.$

Potassium perchloratoborate has density 2.18 $g/cm^3,$ and rubidium perchloratoborate has density 2.32 $g/cm^3.$

The three alkali perchloratoborates fume in moist air, are all crystalline and colourless.

