Bonding orbitals, relative strength

A crude measure of the intrinsic strength of a bond formed using a hybrid orbital, relative to that obtainable using a pure s orbital for a pure p orbital, the figure is 3.00... 

The relative importance of a and r contributions to the overall bonding is unclear, but several different combinations of relative strengths lead to limiting case models. When there are 2 electrons in the forward (T-bond and 2 electrons in the ir-backbond, there are 2 bonding electrons for each metal-carbon bond. This is mathematically equivalent to 2tr-bonds and a metallocyclopropane structure (72). This model does not necessitate strict sp3 hybridization at the carbon atoms. Molecular orbital calculations for cyclopropane (15) indicate that the C—C bonds have higher carbon atom p character than do the C—H bonds. Thus, the metallocyclopropane model allows it interactions with substituent groups on the olefin (68). 

The interaction of C-M bonding orbitals with heteroatom lone-pair orbitals is a two-center-four-electron interaction. This is destabilizing and raises the energy of the HOMO relative to the unperturbed orbitals, and hence lowers the ionization potential of the heteroatom lone-pair electrons. The interaction is qualitatively represented by the molecular orbital interaction diagram shown in Figure 11. The strength of the interactions depends on a number of factors but most importantly on the orientation of the C-M bond with respect to the lone-pair orbital and on the... 

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