Trimethylaluminium

Trimethyl aluminium (TMA) is a pyrophoric colourless liquid. It has a low boiling point, 127 °C, and a high vapor pressure that result in an unpleasant white smoke when the vapor is allowed to react with air. The liquid spontaneously bursts into flames if exposed to air and reacts explosivly with water. An atmosphere of dry nitrogen or argon is required for handling and storage of TMA.

When the aluminium atom forms bonds to three methyl groups, each with a pair of electrons, it is surrounded by six electrons. It is much more stable when surrounded by eight electrons. To achieve this, two TMA units form a dimer joined together by sharing two methyl groups. The shared methyl groups become bridging between the two aluminium atoms. The bridging methyl group contribute two electrons to the bridge bond, and is said to be a 3-center-2-electron bond. Although the carbon of the bridging methyl is surrounden by five neighbors, it is has eight electrons in the outer shell. This electron deficient situation makes TMA very reactive to any substance that may offer electrons. It readily excanges methyl groups for groups and atoms with more electrons to offer, like halogen, which makes it a methylation agent in chemical reactions.

Main uses of TMA is in the production of methylaluminoxane, a activator to polymerization catalysts, as activator on its own, as a methylation agent, and in semiconductor growing.