The Walsh Diagram Pyramidal Methyl

Let s look at the bonding in the planar and pyramidal forms. Orbitals A-C are strongly C-H bonding, whether in the planar or pyramidal forms. They can contain six electrons. In a VB model, we would want three C-H bonds, each with two electrons, for a total of six C-H bonding electrons. The two models agree. With QMOT, we still have three C-H bonds, described by three occupied MOs that are strongly C-H bonding. 

One of the most important concepts in organic chemistry is that of the functional group, a collection of atoms and bonds that shows a consistent reactivity pattern in a wide range of molecular environments. This same notion carries over to electronic structure. Certain collections of atoms contribute a consistent set of orbitals to any molecule in which they are present. To a considerable extent these groups will correspond to the familiar functional groups, but there will be some differences. We will refer to these as group orbitals, a collection of partially delocalized orbitals that is consistently associated with a functional group or similar collection of atoms in molecules. 

The net effect of pyramidalization is thus destabilizing, and so BH3 is predicted to be planar. 

Note that since D or D is empty in BH3, it has no influence on the geometry. 

In contrast, with eight valence electrons, NH3 is predicted to be pyramidal. Now orbital D is occupied in the planar form and D in the pyramidal form, and the substantial stabilization of D associated with pyramidalization outweighs all other considerations. Our simple model has thus made a clear prediction BH3 should be planar, while NH3 should be pyramidal. Moreover, the prediction is correct and is identical to the prediction from VSEPR. 

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