P=O double bond

The present survey is concerned exclusively with short-lived compounds of quinquevalent phosphorus with coordination number 3, which are not yet isolable in a classical sense with few exceptions they all possess at least one P/O double bond. Specifically, these are the metaphosphinates /, the metaphosphonates 2, and the metaphosphates 3. Studies of the methyleneoxophosphoranes 1 (X = O) and the monomeric metaphosphate ion 3 (Z = Oe, X = Y = O) have been especially thorough. 

In Step [2], Ph3P=0 (triphenylphosphine oxide) is eliminated, forming two new n bonds. The formation of the very strong P—O double bond provides the driving force for the Wittig reaction. 

Now the oxide must be removed and this is best done with trivalent phosphorus compounds such as (MeO)3p or PCI3. The phosphorus atom detaches the oxygen atom in a single step to form the very stable P-O double bond. In this reaction the phosphorus atom is acting as both a nucleophile and an electrophile, but mainly as an electrophile since PCI3 is more reactive here than (MeO)3P. 

Anhydrides can contain C—O double bonds or P—O double bonds. (14.1)... 

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