Diphosphene and Phosphaalkene Complexes

Phosphenes (Chapter 8.3) are stabilised by complex formation. The first one to be isolated in this way was parent diphosphene HP=PH in 1974 [2] (8.262a). Complexes of type (8.262a) may, however, be represented by the alternative ring formula (8.262b). By 1990 over 120 diphosphene metal complexes had been made. The first diazene complex (8.262c) was reported in 2008 [3]. 

At least five types of diphosphene complex are known to exist. Either one or both doubly bonded P atoms may participate in o-bonded complexes of types (8.263a) and (8.263b), respectively. Other types such as (8.263c-e) are believed to involve sideways n bonding. 

Complexes of types (8.264a-c) are afforded by chromium carbonyl derivatives of phosphoben-zene, which represent the latter in stabilised form. Two of these can be prepared by reaction (8.265). 

Another stabilised phosphobenzene is represented by the product of reaction 

Bond length measurements on this type of compound indicate double bonds (8.267) as found in stable phosphenes (Chapter 6.19). 

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