Electromeric effect

Although some people refer to it as an effect produced by the presence of a reagent like an electrophile or a nucleophile, IUPAC does not define it as such.

The term electromeric effect is no longer used in standard texts and is considered as obsolete.

This classification is based on the direction of electron pair transfer.

When the attacking reagent is electrophile, the +E effect is generally observed and π-electrons are transferred to the positively charged atom.

When the attacking reagent is a nucleophile, there is generally an -E effect, where π electrons are transferred to atoms to which the attacking reagent will not bind.