Strong molecular orbital model

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. 

In summary, a simple molecular orbital model predicts that formation of a 3c-4e bond between A and BC increases the total bond order from 1 to V2. While this calculation ignores issues of overlap and bond length, it indicates that the A -BC bond in ABC should be roughly 0.4 times as strong as the B-C bond in BC. 

There are definite correlations between bond order, bond energy, and bond length. As the bond order predicted by the molecular orbital model increases, the bond energy increases and the bond length decreases. This is a clear indication that the bond order predicted by the model accurately reflects bond strength, and it strongly supports the reasonableness of the MO model. 

With strong nucleophiles such as methoxide, ring opening follows an Sn2 mechanism. Examine the next to lowest-unoccupied molecular orbital (LUMO+1) for propylene oxide. On which carbon is it most heavily concentrated Is this also the least crowded carbon (Examine a spacefilling model for propylene oxide.) What should be the product of Sn2 addition ... 

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