Pentamethylarsenic
# Pentamethylarsenic

Pentamethylarsenic (or pentamethylarsorane)is an organometalllic compound containing five methyl groups bound to an arsenic atom with formula $As(CH_3)_5.$ It is an example of a hypervalent compound. The molecular shape is trigonal bipyramid.

## History

The first claim to make pentamethylarsenic was in 1862 in a reaction of tetramethylarsonium iodide with dimethylzinc by A. Cahours. For many years all the reproductions of this proved fruitless, so the production proved not to be genuine. It was actually discovered by Karl-Heinz Mitschke and Hubert Schmidbaur in 1973.

## Production

Trimethylarsine is chlorinated to trimethylarsine dichloride, which then reacts with methyl lithium to yield pentamethylarsenic.

    $As(CH_3)_3$ + $Cl_2$ → $As(CH_3)_3Cl_2$
    $As(CH_3)_3Cl_2$ + $2LiCH_3$ → $As(CH_3)_5$ + 2LiCl

Side products include $As(CH_3)_4Cl$ and $As(CH_3)_3=CH_2.$

Pentamethylarsenic is not produced by biological organisms.

## Properties

Pentamethylarsenic smells the same as pentamethylantimony, but is otherwise unique.

The bond lengths in the molecule are for the three equatorial As−C bonds 1.975 Å and the two axial As−C bonds 2.073 Å.

The infrared spectrum of pentamethylarsenic shows strong bands at 582 and 358 $cm^{−1}$ due to axial C-As vibration, and weaker bands at 265 and 297 $cm^{−1}$ due to equatorial C-As vibration. Raman spectrum shows a strong feature at 519, 388, and 113 $cm^{−1},$ and weak lines at 570 and 300 $cm^{−1}.$

## Reactions

Pentamethylarsenic reacts slowly with weak acids. With water it forms tetramethylarsonium hydroxide $As(CH_3)_4OH$ and trimethylarsenic oxide $As(CH_3)_3O.$ With methanol, tetramethylmethoxyarsorane $As(CH_3)_4OCH_3$ is produced. Hydrogen halides react resulting in the formation of tetramethylarsonium halide salts.

When pentamethylarsenic is heated to 100° it decomposes forming trimethylarsine, methane, and ethylene.

When trimethylindium reacts with pentamethylarsenic in benzene solution, a salt precipitates: tetramethylarsenic(V)tetramethylindate(III).

