Organometallic complexes are widely used in chemistry, both in academic research and in industry, ranging from hydrogenation and polymerization reactions, to the production of fine chemicals and semiconductors. However, characterisation of these complexes and the il-lumination of their reaction pathways remains challenging. This is not surprizing because transformations of organometallic complexes in solution often involve the formation of short-lived reaction intermediates present in low concentrations and, therefore, inaccessible to commonly used detection techniques, such as nuclear magnetic resonance (NMR) spectroscopy. Understanding the structure and dynamics of such intermediates is necessary not only for chemical research and catalysis but also for the improved control of nuclear spin states. For example, hydride complexes of Ir and Rh are often employed in hyperpolarization techniques utilizing parahydrogen, a nuclear spin isomer of the hydrogen molecule with the total nuclear spin of zero. In these hyperpolarization techniques, such as parahydrogen-induced polarization (PHIP) and signal amplification by reversible exchange (SABRE), the enriched singlet state of parahydrogen is converted into magnetization of a reaction product or exchanging species. This process results in significant (several orders of magnitude) increase in signal-to-noise ratio (SNR) of NMR signals and allows preparing hyperpolarized molecules for a variety of applications. The iridium complex [Ir(IMes)(H)2(Py)3] + Cl -(1) is extensively used for SABRE hyperpolarization. Recently, experimental evidence has suggested that the evolution of hydrogen spin states in organometallic complexes is strongly influenced by the formation of shortlived intermediates. In particular, it has been suggested that parahydrogen in solution suffers a rapid loss of coherence possibly due to the presence of, until now, unknown reaction intermediates. Such a loss of coherences (also known as singlet-triplet leakage ) is expected to dramatically lower the efficiency of SABRE schemes at high fields. This hypothesis is substantiated by a recent demonstration, that SABRE can be significantly more effective at low magnetic fields, where the singlettriplet leakage is expected to be substantially reduced. 29 This is intriguing because only the dihydride 1 is usually observed in conventional as well as in hyperpolarized room-temperature 1 H NMR experiments, showing that potential intermediates are too short-lived or present in too low concentration for efficient detection under ambient conditions. Thus the quest is out to experimentally search for such intermediates, which could be present after activation of the catalytic precursor [Ir(IMes)(COD)Cl] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazolium, COD = cyclooctadien) in hydrogen atmosphere and under excess substrate (Py = pyridine). In the present paper, we were interested to explore whether it is possible to reveal and characterize such elusive intermediates at room temperature under ambient conditions, employing indirect detection with the Chemical Exchange Saturation Transfer (CEST) experiment.