Past exam of the chemistry B course of the University of Cambridge / 2022 / Paper 2 / B9 / a / ii / Solution

The phosphorus centre is approximately tetrahedral, with three P–C sigma bonds and one short terminal P–O bond. Both $\mathrm{M}{\mathrm{e}}_3\mathrm{P}=\mathrm{O}$ and $\mathrm{M}{\mathrm{e}}_3{\mathrm{P}}^{+}-{\mathrm{O}}^{-}$ are useful Lewis contributors; the bond is strongly polar and has additional multiple-bond character. oxygen lone-pair donation into suitable P–C antibonding combinations provides a description of the additional stabilization without requiring a substantial localized phosphorus-3d/oxygen-2p pi bond. A combined diffraction and orbital study illustrates this sigma-bond/negative-hyperconjugation description.^[ref]

Compared with the phosphate triester, this compound's methyl substituents are less electron withdrawing. The comparison of the two terminal P–O stretching force constants and the appropriate experimental control is given with the triester below.