Trigonal-bipyramidal ML5 complexes

For this geometrical parameter, the differeoce appears to be so small that it can depend on the environment For example, in 

The equatorial and axial ligands are not equivalent by symmetry since no symmetry operation of the complex s point group interchanges an equatorial ligand with an axial. Nor are they equivalent from a chemical point of view, and as a consequence, the M-Leq and M-L ix bond lengths can differ, even when all the ligands L are identical  

With the orientation of the axes chosen in 2-59, the equatorial ligands are located in the xy plane and the axial ligands on the z-axis, that is, at the intersection of the xz and yz planes. 

The five ligands are thus placed in one or other of the nodal planes of thexz (nodal planes ay andyz) andyz (nodal planes xy andxz) orbitals. As a result, there cannot be any a interaction between the ligands and these two orbitals which stay nonbonding in the d-block of a TBP complex (2-60). By consulting the character table for the Daj, point group, we find that these two degenerate orbitals have e symmetry. 

The orbitals shown in 2-62 and 2-63 therefore make up a set of two degenerate orbitals, of e symmetry. Their energy is of course higher than that of the e orbitals xz and yz), since these latter are strictly nonbonding. 

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Trigonal bipyramidal ML5 complexes Table 2.3. Character table for the D3J1 group... 

Why is the structure square-pyramidal It has been shown that a diamagnetic d6 ML5 complex distorts away from the Jahn-Teller active trigonal bipyramidal structure.20 Two more stable structures are possible a square pyramid (T) and a distorted trigonal bipyramid (Y). Theoretical studies21 have shown that the T and Y structures are very close in energy and that the preference for one over the other comes from a subtle balance of a and it properties of the ligands. 

As an example, we will consider a trigonal-bipyramidal complex ML5, in which L is a CT donor only. The point group is and the reducible and irreducible representations are shown here ... 

Crystal field splitting pattern for trigonal bipyramidal (left) and square pyramidal (right) ML5 complexes. 

In terms of applications, the subjects of chemical equivalence and symmetry still deserve honorable mention in that one can use the relative number and NMR multiplicity to assign structure, e.g., trigonal bipyramidal, (7), vs. square pyramidal, (8), in ML5 complexes (especially where L is a spin 7= 1/2 donor). 

Draw qualitative diagrams for the crystal field splittings in (a) a linear complex ion ML2, (b) a trigonal-planar complex ion ML3, and (c) a trigonal-bipyramidal complex ion ML5. 

Fig. 7. The orbital interaction for a trigonal bipyramidal transition metal ML5 complex...

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