Methyl Walsh diagram

Figure 13.2. Walsh diagram for inversion of pyramidal methyl radical...

Figure 7 - Change in total energy and overlap population (left) and Walsh diagram (right) for migration of the methyl group to the vinylic carbon in Pd(PH3)2Me(vinyl).

The methyl group was just a start. Let s now use the same procedure for the CH2 group. Our goals are the same as they were with CH3 define the group orbitals and examine how electron population leads to differing structures. Figure 1.9 shows the Walsh diagram for methylene, where we consider the relationship between the linear and bent forms. 

Methyl radical has seven valence electrons. The crucial orbital that is stabilized by pyra-midalization, aout, is only singly occupied (Figure 1.8), and the stabilization it provides is to some extent offset by the destabilization of other, doubly occupied orbitals. It is difficult to predict the geometry of this species based on the Walsh diagram, and it is best to say that no obvious preference for planar or pyramidal geometry can be predicted. This, too, is "consistent" with experiment, as simple radicals show only a very weak preference for the planar structure, and simple substitution can produce pyramidal radicals. The net result is that the parent methyl radical, CH3, is planar, but the energy cost for distorting away from planarity is small. 

Walsh diagrams for reductive elimination in a three coordinate L-Pd(CH3)2 and L2Pd(CH3)2 complex. For simplicity the L groups were taken to be with their valence state ionization potentials adjusted to match the lone pair orbital of PH3. Here, cp is defined as the C—Pd—C angle this was varied in concert with stretching the Pd—C bonds and rocking the methyl groups off from the Pd—C axis, toward each other. (Reprinted with permission from Reference [65a].)... 

---