Trigonal bipyramidal geometry VSEPR

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

The VSEPR theory tells us that a pentacoordinate phosphorus atom features a trigonal bipyramidal geometry. There are two distinct sites on a trigonal bipyramid, the equatorial and the axial sites. The equatorial sites can be described as utilizing sp -orbitals, and the axial sites the remaining p-orbital. That would mean that the axial bonds have no s-character, and thus feature smaller J coupling constants... 

In addition to the Is and the three 2p orbitals one more orbital is required to make five hybrid orbitals. The closest in energy is the >s orbital. So the hybridization is s p. VSEPR predicts trigonal bipyramidal geometry for five identical substituents. 

For AX molecules with no lone pairs in the valence shell of A, both the VSEPR model and the LCP model predict the same geometries, namely AX2 linear, AX3 equilateral triangular, AX4 tetrahedral, AX5 trigonal bipyramidal, and AX octahedral. Indeed Bent s tangent sphere model can be used equally as a model of the packing of spherical electron pair domains and as a model of the close packing of spherical ligands around the core of the central atom. 

The final cu-bonded formulas (3.213), (3.214), and (3.219)-(3.221) bear an obvious resemblance to the usual VSEPR representations of these hypervalent species. Indeed, each cu-bonded structure has the same number of formal bond pairs (bp) and lone pairs (lp) as the VSEPR representation. Furthermore, the predicted angular geometries of the two models are essentially identical, with the linear (or near-linear) cu-bonded ligands occupying axial positions in the SN2-like trigonal bipyramidal motif. 

This is why SF4 adopts a trigonal bipyramidal as opposed to a tetrahedral equilibrium geometry. Electrons, like atoms, take up space, which is the basis of such models as VSEPR theory. 

Fig. 5. Geometries detected by TAMREAC according to the Angular Overlap and VSEPR models for four- and five-coordinate complexes. Abbreviations SPL = square planar, Td = tetrahedral, SPY = square pyramidal, TBP = trigonal bipyramidal, Is = low spin, hs = high spin, d" = number of electrons in the d configuration of the metal.

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