Phosphorus Halides: PCl3 and PCl5

Phosphorus forms two particularly important chlorides, $\text{PCl}_3$ and $\text{PCl}_5$,

illustrating the group's characteristic $+3/+5$ oxidation-state pair and, in $\text{PCl}_5$'s

case, phosphorus's ability to expand its valence shell beyond an octet using energetically

accessible $3d$ orbitals -- something nitrogen, restricted to its $n=2$ shell, can never do.

Phosphorus trichloride, $\text{PCl}_3$, is prepared by passing dry chlorine gas over gently

heated white phosphorus, using a limited supply of chlorine:

Structurally, $\text{PCl}_3$ is directly analogous to $\text{NCl}_3$/$\text{NH}_3$: phosphorus is

$sp^3$ hybridised, bonded to three chlorine atoms with one lone pair occupying the fourth hybrid

orbital, giving a trigonal pyramidal shape (Cl-P-Cl angle around $100^\circ$). $\text{PCl}_3$

fumes in moist air and is readily and completely hydrolysed by water to phosphorous acid and

hydrochloric acid:

Phosphorus pentachloride, $\text{PCl}_5$, is prepared by treating $\text{PCl}_3$ with further

chlorine, or directly by reacting white phosphorus with an excess of chlorine:

In the gas or vapour phase, $\text{PCl}_5$ is a trigonal bipyramidal molecule: phosphorus

is $sp^3d$ hybridised, with three chlorine atoms occupying equatorial positions at $120^\circ$ to

one another and two chlorine atoms occupying axial positions, perpendicular to the equatorial

plane. The two axial P-Cl bonds are experimentally found to be longer and weaker than the three

equatorial bonds, because each axial bond experiences greater repulsion (from three neighbouring

equatorial bonding pairs at $90^\circ$) than an equatorial bond does (from only two axial pairs

at $90^\circ$ plus two equatorial pairs at a wider $120^\circ$). In the solid state, however,

$\text{PCl}_5$ adopts an entirely different, ionic structure: it crystallises as a lattice of

tetrahedral $[\text{PCl}_4]^+$ cations (phosphorus $sp^3$ hybridised, exactly like the shape of

$\text{PCl}_3$'s conjugate cation, all four Cl at $109.5^\circ$) paired with octahedral

$[\text{PCl}_6]^-$ anions (phosphorus $sp^3d^2$ hybridised, six equivalent Cl at $90^\circ$) -- …