Isomers and electronic states of Ni2O2H2 and evaluation of the effect of charge on the electronic properties and reactivity of Ni2O2.

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 Ni2(OH)2 isomer has a rhombic shape with two OH(-) groups bridging two Ni(I) ions. Its ground term is a (1)Ag term. At a relative energy of 1.06 eV, there is a chain-like NiONi(OH2) isomer. A rhombic (NiH)2O2 isomer with Ni-H bonds has a considerably higher energy of 2.93 eV. Both Ni2(OH)2 and NiONi(OH2) feature a large number of low-lying electronic terms that in the case of Ni2(OH)2 form Heisenberg spin ladders due to the coupling of the electrons of two Ni(I) ions (3d(8)4s(1)) with S = 3/2. For the reaction Ni2O2 + H2 → Ni2(OH)2, the reaction energy is estimated to -2 eV. Finally, neutral and charged Ni2O2 and their hydrogenation products (Ni2O2H2(0/+)) are compared.