When coenzyme B12 was identified as organometallic derivative of vitamin B12, metal-carbon bonds were revealed to be relevant in life processes. Vitamin B12, the “antipernicious anaemia factor” required for human health, was isolated earlier as a crystallizable cyano-Co(III)-complex. B12 cofactors and other cobalt corrinoids play important roles not only in humans, but in the metabolism of archaea and other microorganisms, in particular. Indeed, the microorganisms are the only natural sources of the B12 derivatives. For other B12-requiring organisms the corrinoids are thus “vitamins”. However, vitamin B12 also needs to be converted into organometallic B12-forms, which are the typical coenzymes in metabolically important enzymes. One of these, methionine synthase, catalyzes the transfer of a methyl group and its corrinoid cofactor is methylcobalamin. Another one, methylmalonyl-CoA mutase uses a reversible radical process, and coenzyme B12 (adenosylcobalamin) as its cofactor, to transform methylmalonyl-CoA into succinyl-CoA. In such enzymes, the bound B12 derivatives engage (or are formed) in exceptional organometallic enzymatic reactions, which depend upon the organometallic reactivity of the B12 cofactors. Clearly, organometallic B12 derivatives hold an important position in life and have thus attracted particular interest from the medical sciences, biology, and chemistry. This chapter outlines the unique structures of B12 derivatives and recapitulates their redox properties and their organometallic chemistry, relevant in the context of the metabolic transformation of B12 derivatives into the relevant coenzyme forms and for their use in B12-dependent enzymes.