Hexakis(trimethylphosphine)tungsten

Hexakis(trimethylphosphine)tungsten is a tungsten(0) organometallic compound with the formula W(P(CH3)3)6. It is a yellow crystalline solid soluble in organic solvents.

Synthesis and characterization
Compared to other zerovalent homoleptic trimethylphosphine complexes, W(PMe3)6 was less straightforward to synthesize and isolate. Previous attempts to prepare W(PMe3)6 by co-condensation (a modification of metal vapor synthesis) of tungsten with PMe3 and reduction of WCl6 with alkali metal reducing agents only formed the cyclometalated W(PMe3)4(η2-CH2PMe2)H.

W(PMe3)6 was first isolated in 1990 by Parkin and Rabinovich. It was prepared by the reduction of WCl6 with Na(K) alloy, using PMe3 as a reactive solvent.

Previous attempts to synthesize W(PMe3)6 placed the Na(K) alloy in a glass ampoule in a liquid nitrogen bath, condensed PMe3 into the ampoule, then added WCl6. Following this addition, the ampoule warmed to room temperature and stirred at room temperature for 2 weeks. Parkin and Rabinovich modified this preparation by simply stirring a mixture of WCl6 and Na(K) alloy in PMe3 at room temperature for 10 days.

The complex was crystallographically characterized, demonstrating W-P bond lengths of 2.455 +/- and P-W-P bond angles of 90° and 180°.

W(PMe3)6 demonstrates ΔH° = 9.3 kcal/mol and ΔS° = 37 eu (150 J K-1  mol-1) for the dissociation of PMe3.

Stability
In the solid state, W(PMe3)6 is stable at room temperature for at least two weeks. However, it is unstable in solution, and rapidly converts to an equilibrium mixture between W(PMe3)4(η2-CH2PMe2)H and PMe3 with Keq = 17.8 M at 30°C.

Reactivity
Metal complexes of the form M(PMe3)n contain very electron-rich and highly-reactive metal centers as a result of the combination of the strong σ-donating and π-accepting nature of the PMe3 ligand. These complexes have been shown to be capable of activating C-H and other otherwise unreactive σ-bonds via oxidative addition, often forming cyclometalated products.

W(PMe3)6 possesses an 18-electron valence count, and as such demonstrates somewhat limited reactivity. Much of the most interesting and varied chemistry occurs from its dissociated variants, such as W(PMe3)5.

=== Synthesis of symmetrical and unsymmetrical diphosphenes === W(PMe3)6 can catalyze the metathesis of phosphorus-phosphorus double-bonds. Interaction of the appropriate dichlorophosphane with W(PMe3)6 leads to dechlorination and formation of symmetrical and unsymmetrical diphosphenes.The resultant phosphorus-tungsten species can also catalyze the exchange of diphosphene end-groups.

Formation of metal-germanium triple bonds
The W(PMe3)4(η2-CH2PMe2)H species formed upon the dissolution of W(PMe3)6 can participate in a Ge-Cl bond heterolysis and form a metal-germanium triple bond.