cyanocobalamin—one of the Vitamin B12 family. in these vitamins a cobalt(3) atom is bound to a corrin macrocyclic ligand; this is a ligand containing four pyrrole moieties chained together to form a larger ring, like the porphyrin macrocycles found in biomolecules like heme and chlorophyll, only with 17 atoms in the larger ring rather than 18. the cobalt atom is capable of existing in multiple oxidation states within this complex, which makes the cobalamins valuable catalysts for transferring functional groups to other molecules: one may (for example) bind a methyl group to the Co(3) center to form methylcobalamin, then detach the methyl group and attach it to another molecule by reducing the Co(3) to a lower oxidation state, as shown in the second picture.
this is a general feature of organometallic complexes, especially in biochemistry. a metal atom can be tucked inside a protective scaffolding of organic ligands that permits the metal atom to exist safely in unusual oxidation states that would be highly reactive and dangerous if the metal atom were floating freely in aqueous solution. monovalent cobalt, cobalt(1), is unknown to the routine aqueous chemistry of cobalt, which prefers to exist as Co(2) and Co(3). but in a complex with organic ligands, Co(1) can exist safely and participate in controlled reactions. ~Chara