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Type 8 Lewis Acid/Base Complexation Chemistry

s-LUMO Lewis acids complex poorly with π-HOMO Lewis bases:

• The s-LUMO Lewis acid is "looking for" ligands, the π-HOMO Lewis base provides a large delocalised anionic π-system.

Thus, the bonding interaction is charge controlled and ionic even though the participating species are "looking for" FMO controlled bonding. The problem is that the FMOs are of dissimilar geometry.

Species may only be stable in [non-aqueous] solution where the s-LUMO Lewis acid is complexed by the solvent. Typical solvents are diethyl ether or THF.

s-LUMO Lewis acids are employed as non-electrophilic spectator counter ions to π-HOMO systems with net negative charge: ie the allyl, benzyl and cyclopentadienyl anions. The effect of such complexation is to make the π-HOMO anion to appear ‘naked’ for spectroscopic and reactivity studies.

Bonding in Type 8 complexes is strongly influenced by environment: polar solvents compete with the π-system ligand at complexing the hard s-LUMO Lewis acid cation. s-LUMO/π-anion chemistry is usually carried out in diethyl ether, THF or glyme (polyether) solvents where the ethereal Lewis bases are the true s-LUMO complexing agents.

Most type 8 complexes are strong proton abstracting bases because the conjugate Brønsted acids of the π-HOMO systems are weakly [Brønsted] acidic π-hydrocarbons. The conjugate Brønsted acid of benzyl lithium, for example, is toluene, pKa of 41.

The nature of the s-LUMO cation/π-system bond changes congenerically with cation: a Li⁺/π^– complex is less ionic (therefore more covalent and therefore more strongly ion-paired in solution) than the equivalent Cs⁺/π^– complex.