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My textbook says that in the above diagram that there is only 2 stereoisomers. I am guessing this is due to the stereogenic carbon atom and not due to the phosphorus atom which apparently isn't a stereoisomer.

enter image description here

However, it states that the above molecule has 4 stereoisomers. This must indicate that in this case the phosphorus atom, it is a stereogenic and gives rises to 2 stereoisomers.

Why is the phosphorus atom in A not a stereocenter while the phosphorus atom in B is a stereocenter?

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In compound A, the negative and double bonded oxygens bound to the phosphorus are equivalent:

$\hspace{5.1cm}$resonance

In compound B, they are not equivalent:

$\hspace{7.5cm}$no resonance

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  • $\begingroup$ An even better answer would point out that there isn’t really a $\ce{P=O}$ double bond in that molecule and that $\ce{\overset{-}{O}-\overset{+}{P}R2-\overset{-}{O}}$ would be the better depiction ;) $\endgroup$
    – Jan
    Commented Jan 7, 2017 at 19:06

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