Different isomers of Ni2O2H2 are investigated by multireference configuration interaction (MRCI) calculations, based on complete active space self-consistent field (CASSCF) calculations. The lowest-lying Ni-2(OH)(2) isomer has a rhombic shape with two OH- groups bridging two Ni-1 ions. Its ground term is a (1)A(g) term. At a relative energy of 1.06 eV, there is a chain-like NiONi(OH2) isomer. A rhombic (NiH)(2)O-2 isomer with Ni-H bonds has a considerably higher energy of 2.93 eV. Both Ni-2(OH)(2) and NiONi(OH2) feature a large number of low-lying electronic terms that in the case of Ni-2(OH)(2) form Heisenberg spin ladders due to the coupling of the electrons of two Ni-1 ions (3d(8)4s(I)) with S = 3/2. For the reaction Ni2O2 + H-2 -> Ni-2 (OH)(2), the reaction energy is estimated to -2 eV. Finally, neutral and charged Ni2O2 and their hydrogenation products (Ni2O2H20/+) are compared.