text: The cobalt-containing Vitamin B (also known as cobalamin) catalyzes the transfer of methyl (−CH) groups between two molecules, which involves the breaking of C−C bonds, a process that is energetically expensive in organic reactions. The metal ion lowers the activation energy for the process by forming a transient Co−CH bond. The structure of the coenzyme was famously determined by Dorothy Hodgkin and co-workers, for which she received a Nobel Prize in Chemistry. It consists of a cobalt(II) ion coordinated to four nitrogen atoms of a corrin ring and a fifth nitrogen atom from an imidazole group. In the resting state there is a Co−C sigma bond with the 5′ carbon atom of adenosine. This is a naturally occurring organometallic compound, which explains its function in trans-methylation reactions, such as the reaction carried out by methionine synthase.