The chelate rings made by macrocyclic ligand in vitamin $\mathrm{B}_{12}$ are
(a)One five-membered and three six-membered
(b)Two five-membered and two six-membered
(c)Three five-membered and one six-membered
(d)Four six-membered
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
In the corrin ring one pair of pyrrole units is joined directly (a five-membered chelate ring with Co) and the other three are bridged by methine carbons (three six-membered chelate rings).