Conjugate addition basic principles

The molecular orbital model as a linear combination of atomic orbitals introduced in Chapter 4 was extended in Chapter 6 to diatomic molecules and in Chapter 7 to small polyatomic molecules where advantage was taken of symmetry considerations. At the end of Chapter 7, a brief outline was presented of how to proceed quantitatively to apply the theory to any molecule, based on the variational principle and the solution of a secular determinant. In Chapter 9, this basic procedure was applied to molecules whose geometries allow their classification as conjugated tt systems. We now proceed to three additional types of systems, briefly developing firm qualitative or semiquantitative conclusions, once more strongly related to geometric considerations. They are the recently discovered spheroidal carbon cluster molecule, Cgo (ref. 137), the octahedral complexes of transition metals, and the broad class of metals and semi-metals. 

The NH group lends to pyrrole the functionality of a base as weU as that of an acid [99]. The basicity of pyrrole is very weak (pfQ, = —3.8 for the conjugated acid) compared to cyclic aliphatic amines (e.g., pyrrolidine, pfQ, = +11.3). This large difference is due to incorporation of the nonbonding N-electron pair into the 6it-electron system of pyrrole. In principle, reversible proton addition to pyrrole may occur at all positions, but leads to cations of different thermodynamic stability ... 

---