Principal Transition Metal Derivatives
Triphenylphosphine binds well to most transition metals, especially those in the middle and late transition metals of groups 7-10. In terms of steric bulk, PPh3 has a cone angle of 145°, which is intermediate between those of P(C6H11)3 (170°) and P(CH3)3 (115°). In an early application in homogeneous catalysis, NiBr2(PPh3)2 was used by Walter Reppe for the synthesis of acrylate esters from alkynes, carbon monoxide, and alcohols. Wilkinson's further popularized the use of PPh3, including the then revolutionary hydroformylation catalyst RhH(PPh3)2(CO)2.
It is telling that the corresponding triphenylamine shows little tendency to bind to metals. The difference between the coordinating power of PPh3 and NPh3 reflects the greater steric crowding around the nitrogen atom, which is smaller and favors a more tetrahedral geometry. Far more similar to PPh3 in terms of its coordinating properties is triphenylarsine, AsPh3.
An important technique for the characterization of metal-PPh3 compounds is 31P NMR spectroscopy. Substantial shifts occur upon complexation and31P-31P spin-spin coupling can provide insight into the structure of complexes containing multiple phosphine ligands.
Illustrative PPh3 complexes:
- tetrakis(triphenylphosphine)palladium(0) is widely used to catalyse C-C coupling reactions in organic synthesis, see Heck reaction.
- Wilkinson's catalyst, RhCl(PPh3)3 is a square planar Rh(I) complex of historical significance used to catalyze the hydrogenation of alkenes.
- Vaska's complex, trans-IrCl(CO)(PPh3)2, is also historically significant; it was used to establish the scope of oxidative addition reactions. This early work provided the insights that led to the flowering of the area of homogeneous catalysis.
- NiCl2(PPh3)2 is a tetrahedral (spin triplet) complex of Ni(II). In contrast PdCl2(PPh3)2 is square planar.
- Stryker's reagent, 6, PPh3-stabilized transition metal hydride cluster that used as a reagent for "conjugate reductions."
Read more about this topic: Triphenylphosphine
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