Question

Draw a complete qualitative molecular orbital diagram for CN-. Label everything clearly. Based on the MO diagram and π-bonding theory explain why CN- is a soft base and prefers to bind to soft acids, When CN- reacts with a soft acid, what physical changes in the cyanide molecule would such a reaction lead to?

Answer 1

Drawn below is the detailed MO diagram for CN- species

C has 2s2 2p2 valence electronic configuration

N has 2s2 2p3 valence electronic configuration

Bond order for CN- = 1/2(8-2) = 3

CN- acts as a soft base because here the more electronegative N atom contributes less to the bond formation. The greater electron density around C results in this shift, which makes it behave as soft base. When CN- combines with a sift base, greater overlap of the orbitals occur, giving a stronger bond. Formation of this strong bond is the driving force for the soft base CN- to react with incoming soft acid.