The ground state and the excited state outer electronic configuration of phosphorus (Z=15) are 1s22s22p63s23p3 and 1s22s22p63s13p33d1 respectively.
Now the five orbitals are available for hybridisation to yield a set of five
sp3d hybrid orbitals which are directed towards the five corners of a trigonal bipyramidal. It should be noted that all the bond angles in trigonal bipyramidal geometry are not equivalent . In
PCl5, the five
sp3d orbitals of phosphorus overlap with the singly occupied
p orbitals of chlorine atoms to from five
P−Cl sigma bond. Three
P−Cl bond lie in one plane and make an angle of
120o with each other, these bond are termed as equatorial bonds. The remaining two
P−Cl bonds one lying about and the other lying below the equatorial plane, make an angle of
90o with the plane. These bonds are called axial bonds. s axial bonds suffer more repulsion than equatorial bond, these are found to be slightly longer and hence slightly weaker than equatorial bonds.
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