Singlet and triplet energy surfaces of NiH2

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Abstract
Contracted CI calculations have been performed for the concerted dissociation of NiH2 into Ni and H2. All low‐lying states have been considered. The most important result of the calculations is that the 1A1 state is much lower in energy than the other states for bent geometries. This state even forms a slightly stable complex with an H–Ni–H angle of 49°. The binding is between Ni sd‐hybridized orbitals and a weakened H2 molecule. The ground state of NiH2 is, however, found to be a linear 3Δg state, with the lowest singlet almost 1.5 eV higher in energy at this geometry. The forbidden dissociation reactions for the triplets and singlets with the lowest barriers proceed in steps with the occupation of each symmetry changing by only one unit in each step. The lowest triplet barrier is 1.8 eV, and for this state a concerted dissociation is slightly preferred over a stepwise loss of a hydrogen at a time. The mechanism for the concerted triplet dissociation may also be through spin‐orbit coupling to the 1A1 state, since the states cross. The correlation effects on the potential energy surfaces of NiH2 are further shown to be of qualitative importance.