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

摘要

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.