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.