Vitamin B12, also known as cobalamin, contains a corrin ring as its ligand system.

Here's a breakdown:

* Corrin Ring: This is a macrocyclic ring system that resembles the porphyrin ring found in heme (in hemoglobin). It's composed of four reduced pyrrole rings linked by methine bridges.

* Cobalt Ion: At the center of the corrin ring lies a cobalt ion (Co²⁺). This cobalt ion is the key to B12's function as a cofactor in various enzymatic reactions.

* Axial Ligands: The cobalt ion has two axial coordination sites:

* One site is occupied by a 5,6-dimethylbenzimidazole (DMB) molecule.

* The other site is occupied by a variety of ligands, including methyl, adenosyl, or cyano groups, depending on the specific form of vitamin B12.

The corrin ring with the cobalt ion and its axial ligands is essential for the various enzymatic functions of vitamin B12.