The effect of donation of @E01986@ from filled π or n_π-orbitals to the symmetry adapted σ*-orbital(s) of σ-bond(s) of a molecular entity resulting in building π-character into bonds that nominally possess only σ-character. The consequences of the effect are, for example, in elongation of the C–F bond in the β-fluoroethyl @A00358@ and stabilization of the @A00381@-@T06406@ @C01258@ of fluoromethylamine.