The central atom of these molecules, phosphorus, has only three unpaired electrons out of its five valence electrons as can be seen from its electronic configuration below:
| 1s2 2s2 2p6 3s2 3px1 3py1 3pz1 | ground state configuration |
But the Lewis structures of the given molecules, as shown below, indicate that the phosphorus atom uses five unshared electrons to form five covalent bonds.
It is, therefore, assumed that phosphorus uses the empty d orbital to "promote" 1 electron from 3s orbital to d orbital,
| 1s2 2s2 2p6 3s1 3px1 3py1 3pz1 3dz21 | excited configuration |
and, through hybridization, form five sp3d hybrid orbitals which are then used to bond with chlorine and bromine atoms.
| 1s2 2s2 2p6 (sp3d)1 (sp3d)1 (sp3d)1 (sp3d)1 (sp3d)1 | excited configuration |
Geometry of PCl5
- shape of molecule: trigonal bipyramidal
- bond angle of axial atoms: 180°
- bond angle of equatorial atoms: 120°
- bond angle between an equatorial atom and an axial atom: 90°
Axial atoms are in white circles; equatorial atoms are in black circles.
A trigonal bipyramid is superimposed on the PCl5 molecule.
Geometry of PBr2Cl3
Due to the different axial and equatorial positions the two bromine atoms can assume, PBr2Cl3 (phosphorus dibromo-trichloride) has three geometric isomers as shown below.
Except for the first isomer in which the bromine atoms are in axial positions, the other two geometric isomers of PBr2Cl3 are polar.
