A positive deviation of an -nucleophile (a nucleophile bearing an unshared pair of electrons on an atom adjacent to the nucleophilic site) from a Brønsted-type plot of lg knuc vs. pKa constructed for a series of related normal nucleophiles. More generally, it is the influence of the atom bearing a lone pair of electrons on the reactivity at the adjacent site. HOZ and BUNCEL (1985). See also Brønsted relation.
The use of the term has been extended to include the effect of any substituent on an adjacent reactive centre, for example in the case of the "-silicon effect".