The elusive abnormal CO insertion enabled by metal-ligand cooperative photochemical selectivity inversion.

Direct hydrogenation of CO to CO, the reverse water-gas shift reaction, is an attractive route to CO utilization. However, the use of molecular catalysts is impeded by the general reactivity of metal hydrides with CO. Insertion into M-H bonds results in formates (MO(O)CH), whereas the abnormal insertion to the hydroxycarbonyl isomer (MC(O)OH), which is the key intermediate for CO-selective catalysis, has never been directly observed. We here report that the selectivity of CO insertion into a Ni-H bond can be inverted from normal to abnormal insertion upon switching from thermal to photochemical conditions. Mechanistic examination for abnormal insertion indicates photochemical N-H reductive elimination as the pivotal step that leads to an umpolung of the hydride ligand. This study conceptually introduces metal-ligand cooperation for selectivity control in photochemical transformations.