Type 2: For some heterocyclic directing groups, such as those based on a five-membered heterocycle, TS2 and V are both energetically feasible. Experimentally, these systems build up significant populations of the d1-isotopologue, presumably through substrate exchange via V. These five-membered directing groups are typically quite small. Type 3: For other heterocycle-directed substrates, such as 2-phenylpyridine and 2-phenylpyrimidine, HIE is directed by a six-membered directing group. TS2 is lower in energy than V and so most substrate molecules are doubly deuterated directly after one binding event. Slow substrate exchange, presumably resulting from steric effects, and a lower barrier to phenyldirecting group rotation, promote selectivity for the d2isotopologue. Importantly, these results may also provide some explanation for the lack of correlation between rate and selectivity in iridium-catalysed HIE reactions. If substrate exchange is slow, as is the case for type 3 substrates, then this may account for slow turnover. These results also suggest ways in which control might be exerted over these processes. The rotation of an aryl group while the substrate is bound has a steric demand that is a factor of both the substitution pattern of the substrate, and the steric impact of the NHC and phosphine ligand. For example, recent studies have shown the promise of iridium complexes bearing bidentate cis-chelating ligands for hydrogen isotope exchange, and these have a very different steric profile from systems that bear monodentate NHC and phosphine ligands, such as Ir-1.