 Vitamin B12 is the preeminent bioorganometallic species.Vitamin B12 is actually a collection of related enzyme cofactors, several of which contain cobalt-alkyl bonds, and is involved in biological methylation and 1,2-carbon rearrangement reactions.For a long time since its structure was elucidated by Hodgkin in 1955, it was believed to be the only example of a naturally occurring bioorganometallic system.