 Solution

The larger value for the carbonyl bond means that its dissociation costs $167\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ more for the supplied comparison. A carbonyl combines a strong sigma bond with efficient pi overlap between spatially compatible C 2p and O 2p orbitals. This gives a substantial localized multiple-bond contribution.

The short P–O linkage in a phosphine oxide is also strong, but it is more polar, with a substantial ${\mathrm{P}}^{+}-{\mathrm{O}}^{-}$ contribution. P 3p and O 2p orbitals have less favourable radial and energy matching than C 2p/O 2p for a simple localized pi bond. oxygen donation into P–R antibonding orbitals adds stabilization, but it is not identical to the carbonyl pi bond. Therefore the formal double-bond notation does not imply equal bond strength. A substantial P-3d contribution is not needed to explain the bonding. The stated bond-dissociation values compare complete dissociation processes, so their magnitudes should not be derived solely from one overlap integral or from the two atoms' electronegativity difference.