Reductive dehalogenases are cobalamin-containing enzymes which catalyse the replacement of a halogen atom by a hydrogen atom in organic molecules (X is typically Cl). With reference to the principles of halogen bonding and the properties of cobalamins, propose an outline mechanism for dehalogenation of organic halides starting from Co(I) and suggest how a conserved tyrosine positioned close to the site of reaction may play a key role.