Nucleophile: HS^- : : electrophile : ?

Nucleophiles: ${\mathrm{HS}}^{-}, {\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}^{-},{ \left({\mathrm{CH}}_3\right)}_3\stackrel{··}{\mathrm{N}}, {\mathrm{H}}_2{\mathrm{N}}^{-}$

These species have unshared pair of electrons, which can be donated and shared with an electrophile.

Electrophiles : $\text{BF3, }\stackrel{+}{\text{C}}{\text{l, CH}}_{\text{3}}\text{-}\stackrel{+}{\text{C}}\text{=O, }\stackrel{+}{\text{N}}{\text{O}}_{\text{2}}$

(Reactive sites have only six valence electrons and can accept electron pair from a nucleophile)