Vitamin $B_{12}$ is a coordination compound of _______

Co

The correct answer is Option (4) → Co

Vitamin $B_{12}$ is a naturally occurring coordination compound where the central metal ion is cobalt present inside a large macrocyclic ligand system called corrin ring.

Explanation

Option 1: Cu

Copper forms important biological coordination compounds such as in enzymes like cytochrome oxidase, but it is not the central metal in Vitamin $B_{12}$.

Option 2: Fe

Iron is the central metal in haemoglobin and cytochromes. These are porphyrin-based coordination complexes, not Vitamin $B_{12}$.

Option 3: Mg

Magnesium is the central metal in chlorophyll. It coordinates with a porphyrin-like ring structure involved in photosynthesis.

Option 4: Co

Vitamin $B_{12}$ contains cobalt as the central metal ion. It is present in a corrin ring (similar to porphyrin but slightly contracted). The cobalt ion forms coordinate bonds with nitrogen atoms of the ring and other ligands like $CN^-$ or $CH_3$ depending on the form of Vitamin $B_{12}$. This makes Vitamin $B_{12}$ one of the most important biological coordination compounds.