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If an extended electron rich π-system species reacts via a single atomic centre, for example when an allyl anion is protonated to give propene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-HOMO Lewis base.

However, when when the species reacts via its extended π-system directly, for example during Diels-Alder cycloaddition or when forming a π-organometallic complex, the the species should be considered as a π-HOMO Lewis base species. Thus, there is an overlap between π-stabilised Lobe-HOMO and π-HOMO classification.

Search for π-HOMO Lewis base species in The Chemical Thesaurus

π-HOMO Lewis bases have their electrons in their highest occupied molecular orbital, or HOMO, delocalised over two or more p-orbitals.

Species require modelling by Hückel-FMO techniques as well as by VB-resonance structure methods. Hückel MO modelling gives rise to whole families of π-structure: polyene ribbons, aromatics, etc.:

Indeed, quantum mechanics is all about patterns. A particularly striking manifestation is seen with the polyene system of: 1, 2, 3, 4, 5, 6... conjugated p-orbital systems and how they give rise to the carbanion, allyl anion & pentatrieneyl anion and alkene, 1,3-diene & 1,3,5-triene π-HOMO Lewis bases:

Soft when the entire π-system is acting as the Lewis base, but harder when a single atomic centre is involved and the species is behaving as a Lobe-HOMO Lewis base.

The allyl anion can behave as (or be considered as) 2 π-electrons delocalised over a 3p orbital function, or as a stabilised 2p orbital carbanion. In this latter case it is better to consider the allyl anion to be behaving as a (harder) Lobe-HOMO Lewis base.

Species behave as π-species when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:

• Stabilisation of the π-system: certain patterns/structures are associated with stability such as 4n+2 π-electrons in a cyclic array, the allyl anion, etc.
  
  
• Diels-Alder cycloaddition and other pericyclic interactions, Type 20 Lewis acid/base complexation.
  
  
• Formation of π-organometallic complexes, Type 24 Lewis acid/base complexation.