Transitions trigonal bipyramidal geometry

The MO picture predicts that the reaction will proceed with inversion of configuration because the development of the transition state is accompanied by rehybridization of the carbon to the trigonal bipyramidal geometry. As the reaction proceeds on to product and sp hybridization is reestablished, the product is formed with inversion of configuration. 

A sterically restricted nucleophile is less reactive than a more accessible one because of nonbonded repulsions which develop in the transition state. The trigonal bipyramidal geometry of the 8 2 transition state is sterically more demanding than the tetrahedral reactant, so steric congestion increases as the transition state is approached. 

The vanadium oxide V04 [conjugate base of the acid V03(0H) ] that adopts trigonal bipyramidal geometry and acts as a transition-state mimic for in-hne phosphoryl transfer enzymes. See Oxygen, Oxides, Oxo Anions... 

Fig. 4 Berry pseudorotation for the transition from C4v to Dih symmetry transforming a square pyramidal geometry into a trigonal bipyramidal geometry...

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