Diphosphines, sometimes called bisphosphanes, are organophosphorus compounds used as ligands in inorganic and organometallic chemistry. They are identified by the presence of two phosphino groups linked by a backbone, and are usually chelating.
Many widely used diphosphine ligands have the general formula Ar2P(CH2)nPAr2. These compounds can be prepared from the reaction of X(CH2)nX (X=halogen) and MPPh2 (M = alkali metal):
- Cl(CH2)nCl + 2 NaPPh2 → Ph2P(CH2)nPPh2 + 2 NaCl
Diphosphine ligands can also be prepared by other methods, such as from dilithiated reagents and chlorophosphines:
- XLi2 + 2 ClPAr2 → X(PAr2)2 + 2 LiCl (X = hydrocarbon backbone)
Another popular method involves the addition of secondary phosphines to vinylphosphines:
- Ph2PH + 2 CH2=CHPAr2 → Ph2PCH2-CH2PAr2
Chain length and coordinating properties
The short-chain diphosphine dppm tends to promote metal-metal interactions as illustrated by A-frame complexes. When the two phosphine substituents are linked by two to four carbon centres, the resulting ligands often chelate rings with a single metal. A common diphosphine ligand is dppe, which forms a five-membered chelate ring with most metals.
Some diphosphines, such as the extraordinary case of tBu2P(CH2)10PtBu2, give macrocyclic complexes with as many as 72 atoms in a ring.
To position phosphine donor groups trans on a coordination sphere, several atoms are required to link the donor centres and long-chain diphosphines are typically floppy and do not chelate well. This challenge has been resolved by the long but rigid diphosphine SPANphos. The bite angle of the diphosphine influences the reactivity of the metal center.
Some examples of non-chelating diphosphine also exist. Due to steric effect, these phosphorus atoms can not react with anything except a proton. 
Particularly common diphosphine ligands are shown in the table below:
(from which abbreviation derived)
|IUPAC Name (rarely used)|
|DIPAMP||derivative of phenylanisylmethylphosphine||Ethane-1,2-diylbis[(2-methoxyphenyl)phenylphosphane]|
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