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Concept
Metal-phosphine complex
Summary
A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).
Many metal phosphine complexes are prepared by reactions of metal halides with preformed phosphines. For example, treatment of a suspension of palladium chloride in ethanol with triphenylphosphine yields monomeric bis(triphenylphosphine)palladium(II) chloride units.
[PdCl2]n + 2n PPh3 → n PdCl2(PPh3)2
The first reported phosphine complexes were cis- and trans-PtCl2(PEt3)2 reported by Cahours and Gal in 1870.
Often the phosphine serves both as a ligand and as a reductant. This property is illustrated by the synthesis of many platinum-metal complexes of triphenylphosphine:
RhCl3(H2O)3 + 4 PPh3 → RhCl(PPh3)3 + OPPh3 + 2 HCl + 2 H2O
π-backbonding
Phosphines are L-type ligands. Unlike most metal ammine complexes, metal phosphine complexes tend to be lipophilic, displaying good solubility in organic solvents.
Phosphine ligands are also π-acceptors. Their π-acidity arises from overlap of P-C σ* anti-bonding orbitals with filled metal orbitals. Aryl- and fluorophosphines are stronger π-acceptors than alkylphosphines. Trifluorophosphine (PF3) is a strong π-acid with bonding properties akin to those of the carbonyl ligand. In early work, phosphine ligands were thought to utilize 3d orbitals to form M-P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding. The energy of the σ* orbitals is lower for phosphines with electronegative substituents, and for this reason phosphorus trifluoride is a particularly good π-acceptor.
Image:Connelly-Orpen-R3P-M-sigma-bonding.png|R3P–M σ bonding
File:Connelly-Orpen-R3P-M-pi-backbonding.
Official source
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