Nucleophilic reagents or nucleophiles (nucleus loving) are the reagents possessing at least one lone pair of electrons. Since the nucleophiles are capable of donating electron pairs, they are considered as Lewis bases.

For example,

Nucleophiles carrying an excess of electron pair and also a negative charge( negative nucleophiles) like halide ions X ^- (containing 4 pairs of electrons), hydroxyl ion OH ^-(3 pairs of electrons), alkoxy ion OR^- (3 pairs of electrons),

^-NH₂ (2 pairs of electrons), CN^- (2 pairs of electrons), R ₃C ^-and carbon radicals

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carrying a negative ion (carbanion) COO ^-, ^-CH(COOC₂ H ₅) ₂, CH ₃COCH₂ ,

Nucleophiles rich in eletrons yet electrically neutral like, H-O-H(2 pairs of electrons on O atom), R-O-H (2 pairs of electrons on O atom), R-S-H (2 pairs of electrons on S atom0, R-O-R (2 pairs of electrons on O atom, NH₃ (1 pair on N atom,

R-NH ₂(1 pair on N atom).