NOVELTY - Production of hydrogen involves contacting metal complex (I), with at least one amine substrate (II), or a substrate comprising 2-4 substrates (II) linked via at least one bridging group to form a dimeric, trimeric or tetrameric species, or a substrate comprising 2-4 substrates (II) which are joined to form a fused cyclic dimeric, trimeric or tetrameric species.    USE - In the production of hydrogen use for fuel cell e.g. proton exchange membrane fuel cell (PEMFC), and for dehydrogenating substrates (claimed).    ADVANTAGE - The method allows the controlled and safe release of dihydrogen at a constant rate; and provides catalysts suitable for the catalytic dihydrogen decoupling of ammonia boranes that exhibit improved turnover and/or enhanced reaction kinetics compared to catalysts previously described in the art. The catalyst offers improved long term stability to oxygen and moisture.    DETAILED DESCRIPTION - A process of the production of hydrogen involves: contacting at least one metal complex of formula (I), with at least one amine substrate of formula R21R22NH-BHR23R24 (II), or a substrate comprising 2-4 substrates (II) linked via at least one bridging group to form a dimeric, trimeric or tetrameric species, and where the bridging group is selected from straight or branched 1-8C alkylene optionally substituted by at least one fluoro group; boron; 6-14C aryl; or 6-14C aralkyl; or a substrate comprising 2-4 substrates (II) which are joined to form a fused cyclic dimeric, trimeric or tetrameric species.    X-=anion;    M'=metal selected from Ru, Os, Fe, Co or Ni;    D'=absent, or at least one monodentate or multidentate donor ligand;    Y'=CR13, B or N;    Z1 and Z2==N, =P, NR14, PR15, O, S or Se;    A and B'=(un)saturated or partially unsaturated carbocyclic hydrocarbon ring;    R3 and R4=H, 1-6C alkyl, aryl, 1-6C haloalkyl, or linker group optionally attached to a solid support;or    R3+R4=-C(R16)-Y2-C(R17)-;    Y2=direct single bond or double bond, or CR18;    R1, R2, R5-R13, and R16-R18=H, 1-6C alkyl, 2-6C alkenyl, 2-6C alkynyl, aryl, 1-6C haloalkyl, NR19R20, or linker group optionally attached to solid support;    R14, R15, R19 and R20=H, 1-6C alkyl, 2-6C alkenyl, 2-6C alkynyl, aryl, 1-6C haloalkyl, or linker group optionally attached to a solid support;    R21-R24=H, 1-6C alkyl, fluoro-substituted 1-6C alkyl, 6-14C aryl or 6-14C aralkyl.    The Z2 is a direct bond between carbocyclic ring B and substituent R4. At least two of the R5-R13, and R16-R18 groups are linked, together with the carbons to which they are attached, to form (un)saturated hydrocarbon group. Any two of R21-R24 are linked to form 3-10C alkylene group or 3-10C alkenylene group, which together with the nitrogen and/or boron atoms to which they are attached, forms a cyclic group. INDEPENDENT CLAIMS are included for the following:    (1) new hydrogen generation system comprising at least one of the metal complex (I); at least one of the amine substrate (II), and optionally a solvent;    (2) a fuel cell comprising at least one of the complex (I);    (3) a method of thermolytically dehydrogenating the substrate (II) involving: contacting at least one of the substrate (II) with the complex (I) in the presence of a solvent;    (4) a method of using a hydrogen generation system involving: modulating the hydrogen pressure in the system to modulate activity of the complex (I); and    (5) new metal complexes selected from 26 compounds as given in the specification e.g. metal complexes of formulae (I1)-(I5).