Metal phosphine complexes

In chemistry phosphines are L-type ligands. They are used as ligands for many metal complexes. Perhaps the most popular phosphine ligand used is triphenylphosphine, a shelf-stable solid that undergoes oxidation in air relatively slowly. Unlike most metal ammine complexes, metal phosphine complexes tend to be lipophilic, displaying good solubility in organic solvents. They also are compatible with metals in multiple oxidation states. Because of these two features, 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).

They are both σ-donors and π-acceptors. Phosphine ligands' π-acidity is due to P-C σ* anti-bonding orbitals. Arylphosphines are much stronger π-acceptors than alkylphosphines, which are poor π-acceptors or possibly π-donors. The phosphine with the strongest π-acidity is trifluorophosphine (PF₃); its π-acidity approaches that of the carbonyl ligand.

Wilkinson's catalyst

First generation Grubbs' catalyst bearing tricyclohexylphosphine ligands

Tetrakis(triphenylphosphine)palladium(0), a homoleptic metal phosphine complex

As L-type ligands, phosphines do not change the overall charge of the metal complex. These complexes may be prepared by addition of phosphines to a coordinatively unsaturated metal precursor, or by ligand displacement of another L-type complex, such as a solvent molecule acting as a ligand. For example, palladium chloride forms infinite chains due to the bridging chloride ligands; when treated with triphenylphosphine, the coordination polymer degrades, yielding monomeric bis(triphenylphosphine)palladium(II) chloride units.

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