What is trimethylphosphine?

Trimethylphosphine is an organophosphorus compound with molecular formula, C3H9P or P(CH3)3. The preferred IUPAC name is trimethyl-phosphane and the systematic IUPAC is trimethylphosphorus.

Trimethylphosphine is a colorless liquid with strongly unpleasant odor. The molar mass of trimethylphosphine is 76.079 g/mol.

The melting point of trimethylphosphine is 187K and the boiling point is 312K.

Trimethylphosphine is a weak polar compound with dipole moment 1.2 Debye. It is used as a neutral ligand in co-ordination chemistry.

Structure and bonding of trimethylphosphine

Trimethylphosphine is a derivative of phosphine. The central phosphorus atom is sp3 hybridized and contain one lone pair of electron. Hence it has a pyramidal structure. The C — P — C bond angle is approximately 98.4°.

Since phosphorus atom in tercovalent trimethylphosphine compound has a lone pair of electrons, it behaves as nucleophile in many reactions. It is because of this lone pair act as a good reducing agent.

Preparation of trimethylphosphine

Trimethylphosphine is prepared most conveniently by the reaction of excess of Grignard reagent on phosphorus trichloride or triphenylphosphite.

3CH3MgBr + PCl3 → (CH3)3P + 3MgBrCl

3CH3MgBr + P(OC6H5)3 → P(CH3)3 + 3C6H5-OMgCl

Reactions of trimethylphosphine

Trimethylphosphine is a moderately high basic organophosphorus compound with pKa value 8.65 and hence it reacts with strong acids to give salts. For example, with strong halogen acids, it gives salts with formula, [HPMe3]X.

This reaction is reversible. Because of with strong bases, such as alkyl lithium compounds, a methyl group undergoes deprotonation to give PMe2CH2Li.

Since trimethylphosphine is a reducing agent, it is easily oxidized by air oxygen to trimethylphosphine oxide. It also reacts with methyl bromide to give tetramethylphosphonium bromide.

Uses of trimethylphosphine

Trimethylphosphine is a neutral ligand. The phosphorus atom has a lone pair of electrons through which it can form co-ordinate bond with transition metal atom resulting in the formation of complex compounds.

For this reason, trimethylphosphine is mainly used in co-ordination chemistry. For example, with silver iodide it forms an air stable solid complex with molecular formula, AgI(PMe3).

Toxic character

Trimethylphosphine is toxic and pyrophoric. It converts to a much safer phosphine oxide upon treatment with sodium hypochlorite or hydrogen peroxide.