Our hypothesis was therefore that the competition between these possible processes occurring from VId (substrate exchange vs a second labelling event) might explain our experimental observations regarding labelling selectivity. Free energy profiles, for the steps of interest, for eight of the substrates studied experimentally are shown in Figure . The The free energies of cis-[IrH2(substrate)2(IMes)(PPh3)] + complexes (V) depend acutely on substrate structure. Nitrobenzene and acetophenone lead to complexes that are energetically competitive with, the agostic complexes [IrH2(substrate)(IMes)(PPh3)] + (VI). Conversely, complexes V containing N-heterocyclic directing substrates are considerably higher in energy than the corresponding complexes VI.