Spin delocalisation, theory

E = Si) and EN(R)CH=CHNR (e.g. 2, E = Si) (R = H, Bu ) have been reported " " it was concluded that there is significant p -p -delocalisation for the latter compounds. The relationship between stability, acid-base and spin properties, nucleophilicity and electrophilicity in a series of silylenes was studied by conceptual density functional theory." ... 

The number of configurations required to construct a VB wavefunction of reasonable quality is considerably smaller in approaches implementing the Coul-son-Fischer idea to employ orbitals delocalised over more than one atom. Two of the more widely used modern VB methods, spin-coupled (SC) theory " and the generalised VB (GVB) approach, make full use of this idea by using orbitals constructed as LCAOs, just as in MO theory. Both of these methods do not include any ionic structures in the wavefunction for a neutral system. Other implementations... 

For benzene, the molecular orbital theory states that the six p-orbitals combine to give six molecular orbitals. The three lower-energy molecular orbitals are bonding molecular orbitals, and these are completely filled by the six electrons (which are spin-paired). There are no electrons in the (higher-energy) antibonding orbitals, and hence benzene has a closed bonding shell of delocalised Jt-electrons. 

Texts on alternative types of valence-bond theory include those of references 1 and 2. Reference 3 provides a review of modern ab initio methods and classical valence-bond approaches to electronic structure. References 4 and 5 provide reviews of generalised and spin-coupled valence-bond theory. Both of these theories can use delocalised orbitals that involve more than two atomic centres to accommodate the active-space electrons. In references 6-8, overviews are presented of aspects of increased-valence theory. 

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