Potential energy surfaces of MH2 (M=Co, Fe, and Cu)

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Abstract
The lowest low spin and high spin potential surfaces of MH2, where M is one of the metals Co, Fe, and Cu, have been investigated at the CI level. The results show that CoH2 and FeH2 are very similar to NiH2, which was investigated in an earlier study. On the low spin surface, which is chemically most interesting, H2 dissociates with little or no barrier and forms weakly bound bent complexes. The bonds are formed from sd hybrids. These results are in line with experimental findings for dissociative chemisorption of H2 on metal surfaces, where Fe, Co, and Ni are known to behave very similarly. Since sd hybrids for Cu can not be easily formed the addition of H2 to Cu leads to a high barrier, which means that on-top dissociation of H2 on a copper surface is unlikely. This is a plausible explanation of why H2 dissociates with higher barriers on copper surfaces than on iron, cobalt and nickel surfaces. In gas phase, the ground states of MH2 are linear with high spin. The ordering of these linear states is explained using very simple rules. The high barrier dissociations of the high spin states are also discussed.