Poly phosphine Multidentate Ligands

In addition to the bidentate ligands depicted in Table 6.15, there are tripod-type ligands (below) and many other multidentate poly(phosphine) ligands which can form complexes. Those based on phenyl groups seem to be the most popular (Chapter 6.17). Poly(phosphine) ligands are useful in stabilising metal cluster complexes (above) and cyclic and acyclic P units. 

When carbon chain poly(phosphines) act as multidentate ligands they form chelate rings containing carbon, phosphorus and a metal atom. They can be made by reactions analogous to those employed for monodentate derivatives above. In reaction (8.137) a linear complex is first formed, but on raising the temperature a cyclic derivative is obtained. 

A typical series of bidentate compounds are (8.138). While chain length does not appear greatly to affect bond angles at the P atom, it may decide between cis or trans isomer formation. Thus when molybdenum hexacarbonyl reacts with a two-carbon phosphine the trans compound (8.139a) is formed, but with a single carbon phosphine the cis derivative (8.139b) is obtained. 

Some carbon chain polyphosphines form complexes with very large rings (8.142). These may dissociate in solution, but their stability appears to be related to the nature of the groups attached to the P atom. 

Compounds (8.143b,c) are of interest because the octahedral rhodium complex will take up oxygen reversibly whereas with the octahedral iridium complex the process is irreversible. The latter 

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