Canonical molecular orbital mixings

However, the division of the electron density at the iron nucleus into contributions arising from Is through 4s contributions can be done conveniently at the level of the canonical molecular orbitals. This arises because the iron Is, 2s, and 3s orbitals fall into an orbital energy range where they are well isolated and hence do not mix with any hgand orbital. Hence, the Is, 2s, and 3s contributions are well defined in this way. The 4s contribution then arises typically from several, if not many, molecular orbitals in the valence region that have contributions from the iron s-orbitals. Thus, the difference between the total electron density at the nucleus and... 

Whereas the localized NBO description shows the clear distinction between conjugated and unconjugated double bonds, this distinction is far from obvious in the canonical MO picture. For example, the highest occupied molecular orbitals (HO-MOs) are found to have superficially similar strongly mixed forms in 1 and 9 ... 

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