sp³d Hybridization

This Hybridization involves the mixing of one s, three p and one d-orbital. These five orbitals hybridize to form five sp³d-hybrid orbitals. The mixing of five orbitals is shown in figure 1.23. These hybrid orbitals point towards the corners of a trigonal bipyramidal geometry. In this case, the three orbitals forming a plane are directed towards the corners of an equilateral triangle while the other two are perpendicular to the plane of the triangle lying above and below it.

fig 1.23 - Formation of five sp³d hybrid orbitals, which adopt trigonal bipyramidal geometryPhosphorus pentafluoride involves sp³d-Hybridization as described below:

Geometry of PF

₅ molecule

The outer electronic configuration of phosphorus, the central atom, is 3s² 3p³ which may be represented as shown in fig.1.24 (a). It has three unpaired electrons in the ground state:

fig 1.24 - Formation of PF₅ molecule involving sp³d Hybridization

To explain the pentavalency of phosphorus in PF₅, one of the electrons in 3s orbital is promoted to the higher 3d orbital so that it has five unpaired electrons. This is called the excited state of phosphorus and is represented in fig.1.24 (b). These five orbitals hybridise to form five sp³d hybrid orbitals, which adopt trigonal bipyramidal arrangement. Each of the sp³d hybrid orbitals overlaps with 2p orbital of fluorine forming five P-F bonds as shown in fig.1.25.

fig 1.25 - Structure of PF₅

Thus, PF₅ molecule has trigonal bipyramidal geometry. It may be noted that all the bond angles in trigonal bipyramidal geometry are not equivalent. Three bonds lie in one plane at an angle of 120^o to one another as in a triangular planar arrangement. These bonds are termed as equatorial bonds. Of the remaining two bonds, one lies above and other below the equatorial plane both making an angle of 90^o with the plane. These bonds are called axial bonds. It may be remembered that this geometry is not symmetrical. The axial bonds have been found to be larger than equatorial bonds. For e.g., in case of PF₅ molecule each P - F axial bond = 219 pm and each P - F equatorial bond = 204 pm. As the axial bonds suffer more repulsion,the axial bond will be slightly elongated when compared to equatorial bond.