Solution

Vitamin $B_{12}$ (cyanocobalamin) is a coordination compound of cobalt.
The cobalt is held in a corrin ring (similar to a porphyrin ring) by four nitrogen atoms, with a cyanide group and a nitrogen base as the other two ligands.
Cobalt is in the +3 oxidation state, Co(III).
Iron is present in haemoglobin, magnesium in chlorophyll, zinc in carbonic anhydrase.
So vitamin $B_{12}$ contains Co(III).

Concept Cobalt in cyanocobalamin

Coordination Compounds in past papers

104 questions from this chapter have appeared across 27 exam years.