Ground-State Isomerization Berry Pseudorotation

The hybridization traditionally assumed for the trigonal bipyramid is sp d, the configuration in the initial geometry being that of the constituent AOs 3s a y,3pz a y, (3px, 3py)(e ), 3d2 .2 a-2 y2(a ) ). It can be seen on the left side of Fig. 11.1 that the configuration based on the five combinations of halide-ion closed shells is identical. From the point of view of configurational symmetry, inclusion of a d orbital in the hybridization scheme in order to produce five equivalent covalent bonds is harmless but unnecessary. 

This conclusion is in accord with detailed ab initio calculations [8], according to which C2V symmetry is retained along the pseudorotation pathway of PF5. The calculated transition state, in which the lengths of the bonds to F2, F3, F4 and F5 are equal and the symmetry has risen to C4V, lies 4 kcal/mol above the ground state, in good agreement with experiment. 

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