Which can not behave as a nucleophile for S_ N 2 reaction?

In $\mathrm{S}_{\mathrm{N}} 2$ reaction, nucleophile transfers electron pair to carbon of substrate, leading to the formation of transition state. $\mathrm{H}_2 \mathrm{O}$ is a neutral nucleophile while $\mathrm{CN}^{-}, \mathrm{NH}_2^{-}$and $\mathrm{I}^{-}$are negatively charged nucleophile. Thus, $\mathrm{H}_2 \mathrm{O}$ cannot behave as a nucleophile for $\mathrm{S}_{\mathrm{N}} 2$ reaction.