Many structures of phosphorus-containing compounds are drawn...

Many structures of phosphorus-containing compounds are drawn with some $\mathrm{P}=\mathrm{O}$ bonds. These bonds are not the typical $\pi$ bonds we've considered, which involve the overlap of two $p$ orbitals. Instead, they result from the overlap of a $d$ orbital on the phosphorus atom with a $p$ orbital on oxygen. This type of $\pi$ bonding is sometimes used as an explanation for why $\mathrm{H}_{3} \mathrm{PO}_{3}$ has the first structure below rather than the second: Draw a picture showing how a $d$ orbital and a $p$ orbital overlap to form a $\pi$ bond.

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Key Concepts

Pi Bonding

Pi bonding refers to the overlap of orbitals in a side-by-side manner, which occurs above and below the plane of the nuclei of the bonding atoms. This type of bonding is responsible for the double or triple bonds in molecules, adding extra stabilization beyond the sigma bond, which is formed by head-on overlap.

d Orbital Participation in Bonding

Elements like phosphorus can utilize available d orbitals to form bonds, especially in molecules that appear hypervalent. In certain phosphorus compounds, a d orbital on the phosphorus atom overlaps with a p orbital on an oxygen atom to form a bond resembling a pi bond, thus extending the typical concept of bonding beyond p orbital interactions.

Orbital Overlap and Symmetry

For an effective bond to be formed, the interacting orbitals must have the appropriate symmetry and spatial orientation. In the context of the P=O bond, the alignment and phase of the d orbital on phosphorus and the p orbital on oxygen allow for the lateral overlap necessary to create a pi bond, demonstrating the importance of orbital shapes and symmetry in chemical bonding.

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