Isomerism of Phosphaethynes and Diphosphenes

Quite a few phosphaethynes have now been synthesised (Chapter 6.14). Unsymmetrically substituted derivatives do exist in syn and anti isomeric forms (13.81a) just as the longer-known ketox-imes (13.81b). 

Fractional crystallisation and chromatographic methods have been used to isolate the two isomers (13.82) in crystalline form [29,30]. The two forms remain stable in solution even at elevated temperatures, but irradiation produces a mixture. 

Three forms of the diphosphabutadiene (13.83) have been observed  

Phosphine-substituted derivatives of ethylene are known in syn and anti forms. In this case the two isomers can be prepared by the action of LiPPh2 on the appropriate ethylene derivative. 

Isomeric cis and trans phosphobenzene derivatives (13.86a), analogous to cis and trans azobenzene (13.86b) can be isolated. 

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