Pyramidal ML3 complexes

These entities are interesting since they may be considered as fragments of more important complexes, such as octahedral or square-planar. In this context, knowledge of their orbital structure is useful for two reasons  

They can be used as fragment orbitals in some complexes. For example, the main MO of the complex [( -cyclopentadienyl)Mn(CO)3] (2-76) can be constructed by interaction of the 7t MO of cyclopentadienyl with the d orbitals of the metallic fragment [Mn(CO)3] in a pyramidal geometry. 

The d orbitals of these species resemble the orbitals of certain organic molecules or fragments, both in their shape and their electronic occupation. It is therefore important to be familiar with them if one wishes to establish a link between the electronic structures of transition metal complexes and of organic molecules (the tsolobal analogy, see Chapter 5). 

In one approach to the derivation of the d-hlock orhitals of this complex, we start from an octahedral complex with the geometry indicated in 2-77 and remove the three ligands L4, L5, and L. A pyramidal ML3 fragment is thereby obtained, in which the angles between the M L bonds are 90°. In the orientation that is chosen, the z-axis coincides with the C3 axis of the resulting ML3 complex. 

Notice that the structure of the d block, with three strictly nonbonding orbitals, enables us to imderstand why this geometry is observed for low-spin d complexes, such as that represented in 2-75. 

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