NOVELTY - An iron-sulfur complex (I) or (II) is new.    USE - New iron-sulfur complex is used as catalyst for producing hydrogen gas (all claimed) used as fuel.    ADVANTAGE - The iron-sulfur complex has excellent catalytic efficiency.    DETAILED DESCRIPTION - An iron-sulfur complex of formula (I) or (II), is new.    z=-3 to +2;    L1-L3,L5,L6,X1,X2=alkyl, alkenyl, alkynyl, phenyl, benzyl, hydroxy, carbonyl, aldehyde, haloformyl, carbonate ester, carboxylate, carboxy, ester, methoxy, hydroperoxy, peroxy, ether, hemiacetal, hemiketal, acetal, ketal, orthoester, orthocarbonate ester, carboxamide, primary amino, secondary amino, tertiary amino, primary ketimino, secondary ketimino, primary aldimino, secondary aldimino, imido, azido, azo, cyanato, isocyanato, nitrate, nitrile, isonitrile, nitrosooxy, nitro, nitroso, pyridyl, sulfhydryl, sulfide, disulfide, sulfinyl, sulfonyl, sulfino, sulfo, thiocyanato, isothiocyanato, carbonothioyl, phosphino, phosphono, phosphate, borono, boronate, borino, borinate or halo;    L4=bridging ligand chosen from hydroxy, carbonyl, cyanide, primary amide, secondary amide, sulfide, disulfide, sulfinyl, thiolate, phosphide, halide, oxide, nitride, borylene, boryl, boride and hydride;    X3=vacant site or optionally substituted X2;    z1,z2=-3 to +2 (excluding 0);    (A)(power(z2))=counter ion chosen from K+(L7)n, Na+(L7)n, Li+(L7)n, (n-Bu4N)+, (PPN)+ or (CF3SO3)-;    L7=coordinating organic molecule chosen from 18-crown-6-ether (1,4,7,10,13,16-hexaoxacyclooctadecane), 15-crown-5-ether (1,4,7,10,13-pentaoxacyclopentadecane), 12-crown-4-ether (1,4,7,10-tetraoxacyclododecane), or dibenzo-18-crown-6 (2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene), or coordinating solvent molecule chosen from H2O, tetrahydrofuran or MeCN;and    n=+1 to +13.    An INDEPENDENT CLAIM is included for method for producing hydrogen, which involves providing an electric potential to a proton-source substance in presence of iron-sulfur complex (I) or (II) as a catalyst such that proton in the proton-source substance is reduced to form hydrogen gas.