A Redox Non-Innocent Ligand Controls the Life Time of a Reactive Quartet Excited State - An MCSCF Study of [Ni(H)(OH)]+

Abstract

The electronic structures of the low and high-spin states of the cationic complex [Ni(H)(OH)](+) that was previously found to be highly reactive toward CH(4) and O(2) were examined. Earlier computational work suggested that the low-spin doublet state D(0) of the Ni(III)-d(7) system is significantly lower in energy than its high-spin quartet analogue Q(1). Recent DFT-studies indicated, however, that Q(1) is the reactive species requiring Q(1) to have a sufficiently long lifetime for undergoing thermal reactions with the small molecule reactants under single collision conditions in the gas phase. These observations raise the question as to why Q(1) does not spontaneously undergo intersystem crossing. Our work based on DFT, coupled-cluster and MCSCF calculations suggests that the hydroxyl ligand behaves as a redox noninnocent ligand and becomes oxidized to formally afford an electronic structure that is consistent with a Ni(II)-(OH)* species. As a result, the doublet and quartet ground states are not related by a single electron spin flip and the intersystem crossing becomes inhibited, as indicated by unexpectedly small spin-orbit coupling constants. After extensive sampling of the potential energy surfaces, we concluded that there is no direct way of converting Q(1) to the ground state doublet D(0). Alternative multistep pathways for the Q(1) --> D(0) decay involving doublet excited states were also evaluated and found to be energetically not accessible under the experimental conditions.