Pi bond order

As a general rule, pi bond orders, pi overlap populations and total pi overlap populations allow a diagnosis of the type of pi interactions which obtain between two AO s. The situation is much more uncertain in the case of sigma AO interactions, and one has to rely upon bond orders and/or overlap populations. 

Similarly, orbital interaction diagrams can also explain the stabilization of a radical center by a pi donor (Fig. 11.3fc). The pi donor has an accessible full orbital, HOMO, close in energy to the orbital bearing the single electron, SOMO. The interaction of the SOMO with a full HOMO destabilizes one electron and stabilizes two for a net stabilization of one electron overall. We have created a pi bond with one electron in the antibonding orbital the pi bond order is thus half (our resonance structures were unable to indicate this partial pi bond with lines and dots). 

Here P, and D, represent the degree of imbalance of valence electrons in the p and d orbitals on atom 4. P, may be written in terms of orbital populations or in terms of qualitative constants such as ionicity of the bond, pi bond order, degree of s character ... 

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