Isomerism of Polyphosphine Derivatives

It has long been recognised by organic chemists that if a compound contains two asymmetric carbon atoms, internal compensation is possible. The likelihood of meso or internally compensated molecules arises in unsymmetrically substituted diphosphines, hypophosphates, pyrophosphates and so on. 

Diphosphine, P2H4, can conceivably exist in three different conformations (13.88). Such isomers can be interconverted by rotation about the P-P bond and are known as rotational isomers or rotam-ers. The gauche form of P2H4 is most favoured by spectroscopic evidence, while P2Me4 and the halides P2X4 (X = F,C1 or Br) appear to exist in trani-conflguration in both solid and liquid states. 

There are numerous possibilities for both positional and optical isomerism amongst the polyphosphines (Chapter 4.4), and the existence of many examples of these has been demonstrated by NMR techniques. There are, for example, two positional isomers of tetraphosphine, P4Hg. 

The normal form of P4H5 (13.89a) has three isomeric forms (13.90) (the lone pair electrons on the P atom can be regarded as completing an asymmetrical tetrahedral environment). Evidence for the existence of numerous isomers amongst higher polyphosphines has now accumulated. 

In the case of substituted diphosphines, the possibility of positional, rotational and optical isomerism arises. With the compound Mc2P-PPh2, cis, trans and gauche forms are conceivable. Interconversion of the non-opticaUy active cis and trans forms can be effected either by rotation about the P-P bond or by inversion of the pyramidal configuration about one of the P atoms. 

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