# Is "CN"^(-) a Lewis base? Why?

By definition, a $\text{Lewis base}$ is an electron pair donor..and certainly cyanide anion acts as a ligand in coordination chemistry. Cyanide is isolectronic with $C O$, and while not as $\pi - \text{acidic}$ as the carbonyl can accept some electron density from the metal centre.
""^(-)C-=N+H_2O(l)rightleftharpoonsHC-=N(aq) + HO^(-)