The ability of nickel(II) macrocycle and coordination complexes 1-8 to mediate the reductive dehalogenation of cyclohexyl bromide and the CH3-S bond cleavage of methyl CoM by sodium borohydride in diglyme/alcohol, DMF/alcohol, or acetonitrile/alcohol was investigated. Methyl CoM, or CH3SCH2CH2SO3-, is the cofactor that carries the methyl group in the final step of methanogenesis in methanogenic bacteria. Because of the potential for production of the heterogeneous catalyst nickel boride during these reactions, the activities of several nickel salts that afford nickel boride when reduced with borohydride were examined for purposes of comparison. Complexes 1-8 homogeneously catalyze the dehalogenation of cyclohexyl bromide by sodium borohydride. The facility of the reaction varies markedly with the structure of the ligands and the solvent composition. Nickel boride is a moderately active heterogeneous catalyst for the dehalogenation of cyclohexyl bromide and produces small yields of methane from methyl CoM and borohydride. When excess nickel boride is generated in situ, the yield of methane increases to 54%. The other isolated products, ethanesulfonate and a product derived from the CH3-S part of methyl CoM, show that nickel boride preferentially cleaves the CH2-S bond of methyl CoM, which is the opposite of the enzymatic selectivity. Freshly prepared Raney nickel, a second heterogeneous nickel compound, quantitatively cleaves methyl CoM to methane and ethanesulfonate. None of the complexes 1-8 produced significant amounts of methane from methyl CoM and sodium borohydride in the mixed solvents or in aqueous solution.