The report that the nickel(II) complex of the macrocyclic ligand 1,4,7,10,13-pentaazacyclohexadecane-14,16-dione, L, cleaves CH3SCH2CH2SO3- to CH4 and HSCH2CH2SO3- was reexamined. CH3SCH2CH2SO3-, or cofactor methyl coenzyme-M, carries the methyl group in the final step of methanogensis in methanogenic bacteria. The cleavage of the cofactor was reproduced when the nickel complex of L synthesized from unpurified, technical grade tetraethylenepentamine was used. However, authentic samples of the nickel complex were found to be incapable of carrying out the cleavage reaction. NiL(OAc)(2) prepared from L synthesized from pure tetraethylenepentamine crystallizes in the monoclinic space group P2(1)/c (Z = 4) with unit cell dimensions a = 8.234(1) Angstrom, b = 13.439(2) Angstrom, c = 18.915(2) Angstrom, beta = 95.370(10)degrees, and V = 2083.9(5) Angstrom(3).