DFT studies provided further information on the relationship between structure and reactivity. [IrH2(IMes)(PPh3) (substrate)2] + complexes (V) provide a route for the exchange of Scheme 3 Summary of mechanistic hypothesis and rationale for why the d2-isotopologue is produced more quickly for 2-phenylpyridine and 2-phenylpyrimidine. d0/d1/d2-substrate, while transition states (TS2) were obtained for rotation of the phenyl group on bound substrates, which can facilitate double ortho-C-H deuteration after a single binding event. For those substrates that lead to higher initial d2isotopologue populations, the rotation around the C-C bond linking the phenyl group and the directing group is more favourable than the coordination of a second molecule of substrate.