Mechanism A: During the reaction with H2, CO must dissociate from 2a to ultimately form the product 3a. Dissociation could potentially occur prior to, or after, the H2 splitting step. The order of these events was probed through inhibition and isotope labelling experiments. The addition of 1:1 mixture of CO:H2 to a solution of 2a resulted in a complete inhibition of H2 activation, even under the exposure of LED light over 24 hours. Reaction of 2a with CO demonstrated that ligand exchange was possible at 25 °C after 20 h. This reaction was quicker under photochemical conditions and complete in 2 h. These experiments support the facile and reversible dissociation of CO from 2a under photochemical conditions and based on these findings it is likely this occurs before H2 activation step. Dissociation of CO from 2a would generate a coordinatively unsaturated 16-electron intermediate that has the potential to directly react with H2 by an oxidative addition step.