Trimethylphosphine

Trimethylphosphine
Preferred IUPAC name Trimethylphosphane
Systematic name Trimethylphosphane[1] (substitutive) Trimethylphosphorus[1] (additive)
Identifiers
CAS number	594-09-2 Y
PubChem	68983
ChemSpider	62205 N
EC number	209-823-1
UN number	1993
MeSH	trimethyl+phosphine
ChEBI	CHEBI:35890 N
Beilstein Reference	969138
Jmol-3D images	Image 1
SMILES CP(C)C
InChI InChI=1S/C3H9P/c1-4(2)3/h1-3H3 N Key: YWWDBCBWQNCYNR-UHFFFAOYSA-N N
Properties
Molecular formula	C3H9P
Molar mass	76.08 g mol−1
Appearance	Colorless liquid
Density	735 mg cm-3
Melting point	−86 °C; −123 °F; 187 K
Boiling point	38 to 39 °C; 100 to 102 °F; 311 to 312 K
Vapor pressure	49.9 kPa (at 20 °C)
Structure
Coordination geometry	Trigonal pyramidal
Dipole moment	1.19 Debye
Hazards
GHS pictograms
GHS signal word	DANGER
GHS hazard statements	H225, H315, H319, H335
GHS precautionary statements	P210, P261, P305+351+338
EU classification	F Xi
R-phrases	R11, R36/37/38
S-phrases	S9, S16, S26, S36/37/39
Flash point	−19 °C; −2 °F; 254 K
Related compounds
Related compounds	NMe3 PH3 PPh3
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Trimethylphosphine is the organophosphorus compound with the formula P(CH₃)₃, commonly abbreviated as PMe₃. This colorless liquid has a strongly unpleasant odor, which is characteristic of alkylphosphines. The compound is a common ligand in coordination chemistry.

Structure and bonding

It is a pyramidal molecule with approximate C_3v symmetry — the molecular point group of ammonia (NH₃) and phosphine (PH₃). The C–P–C bond angles are approximately 98.6°.^[2]

The C–P–C bond angles are consistent with the notion that phosphorus predominantly uses the 3p orbitals for forming bonds and that there is little sp hybridization of the phosphorus atom. The latter is a common feature of the chemistry of phosphorus because the gap between the 3s and 3p orbital is large compared to the corresponding gap for the 2s and 2p orbitals that are in the valence shell for carbon, nitrogen, and oxygen (elements for which sp-mixing is strong). As a result, the lone pair of trimethylphosphine has predominantly s-character as is the case for phosphine, PH₃.^[3]

PMe₃ is usually be prepared by the treatment of triphenyl phosphite with methylmagnesium chloride:^[4]

3 CH₃MgCl + P(OC₆H₅)₃ → P(CH₃)₃ + 3 C₆H₅OMgCl

The synthesis is conducted in dibutyl ether, from which the more volatile PMe₃ can be distilled.

Reactions

Acid-base reactions

With a pKa of 8.65, PMe₃ reacts with strong acids to give salts [HPMe₃]X.^[2] This reaction is reversible. WIth strong bases, such as alkyl lithium compounds, a methyl group undergoes deprotonation to give PMe₂CH₂Li.

PMe₃ is easily oxidised to the phosphine oxide with oxygen. It is also easily alkylated to give phosphonium derivatives RPMe₃⁺. The compound is stable to water.

Coordination chemistry

Trimethylphosphine is a highly basic ligand that forms complexes with most metals. As a ligand, trimethylphosphine's Tolman cone angle is 118°.^[5] This angle is an indication of the amount of steric protection that this ligand provides to the metal that to which it is bound.

Being a relatively compact ligand, several can bind to a single transition metal, as illustrated by this Fe(0) complex:

2 PMe₃ + Fe(CO)₅ → Fe(CO)₃(PMe₃)₂ + 2 CO

Its complex with silver iodide, AgI(PMe₃) is an air-stable solid that releases PMe₃ upon heating.

Safety

PMe₃ is potentially pyrophoric as well as toxic. PMe₃ converts to non-pyrophoric phosphine oxide by treatment with dilute bleach.

References