Abstract: The present invention relates to a process for the production of hydrogen comprising contacting at least one complex of formula (I), (I) wherein: X? is an anion; M is a metal selected from Ru, Os, Fe, Co and Ni; D is optionally present and is one or more monodentate or multidentate donor ligands; Y1 is selected from CR13, B and N; Z1 and Z2 are each independently selected from ?N, ?P, NR14, PR15, O, S and Se; or Z2 is a direct bond between carbocyclic ring B and substituent R4; each of A and B is independently a saturated, unsaturated or partially unsaturated carbocyclic hydrocarbon ring; R3 and R4 are each independently selected from H, C1-6-alkyl, aryl and C1-6-haloalkyl, and a linker group optionally attached to a solid support; or R3 and R4 together form the following moiety: (AB) Y2 is a direct single bond or double bond, or is CR18; R1, R2, R5-13 and R16-18 are each independently selected from H, C1-6-alkyl, C2-6-alkenyl, C2-6-alkynyl, aryl, C1-6-haloalkyl, NR19R20 and a linker group optionally attached