For the determinisation of the lifetimes of the singlet state in the complexes[Ir(IMes)(H)2(Py)3] + PF6 -( ) and [Ir(Cl)(IMes)(H)2(Py)2] (2) two different samples were prepared according to the description provided above, where one of the forms is the dominant one. Figure shows a spectrum of a solution of benzene, where 1 is the dominant form. The hydrides of species 2 are only visible in hyperpolarized spectra. For the determination of the singlet state lifetime, OPSY measurements with and without spinlocking of the hydrides were performed as described in the main text. The signal evaluated for the lifetime measurements was the hyperpolarized species of the hydrides of 2 as the chemically equivalent hydrides of 1 are not visible when the OPSY coherence filter is applied.