Available online at www.sciencedirect.com Coordination Chemistry Reviews 252 (2008) 2278–2291 Review Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry Simon B. Duckett ∗ , Nicholas J. Wood Department of Chemistry, University of York, Heslington, York YO10 5DD, UK Received 5 November 2007; accepted 24 January 2008 Available online 3 February 2008 Contents 1. Introduction ........................................................................................................... 2278 1.1. The existence of spin-isomers of hydrogen ......................................................................... 2279 1.2. Parahydrogen-enhanced NMR .................................................................................... 2279 1.3. Experimental considerations ...................................................................................... 2280 1.4. The scope of this review .......................................................................................... 2281 2. The use of parahydrogen in inorganic chemistry ........................................................................... 2281 2.1. The detection and investigation of metal hydride species ............................................................. 2281 2.2. Studies of catalytic hydrogenation with parahydrogen ............................................................... 2285 2.3. Studies of catalytic hydroformylation with parahydrogen ............................................................ 2288 2.4. In situ photochemistry with parahydrogen .......................................................................... 2288 3. Conclusions ........................................................................................................... 2289 Acknowledgements .................................................................................................... 2289 References ............................................................................................................ 2289 Abstract The study of reactions by NMR spectroscopy is normally limited by the poor detection limits offered by the method. An overview is presented of how chemical reactions can be studied using parahydrogen-assisted NMR spectroscopy, where detected signals can have strengths that exceed those normally available by factors that approach 31,000. © 2008 Elsevier B.V. All rights reserved. Keywords: NMR; Parahydrogen; Mechanism; Photochemistry; Hydrogenation; Catalysis 1. Introduction Many metal complexes react with molecular hydrogen, form- ing complexes with either discrete  2 -H 2 moieties or ‘classical’ metal-hydride ligands. NMR spectroscopy is often a convenient way to monitor these reactions and characterise their products because the 1 H nuclei of these ligands generally resonate in a characteristically high field region of the NMR spectrum. How- ever, if the hydrogen addition reaction is unfavourable, or if the product is rapidly consumed in a subsequent reaction, then ∗ Corresponding author. Tel.: +44 1904 432564; fax: +44 1904 432516. E-mail address: sbd3@york.ac.uk (S.B. Duckett). the species will exist only in low concentration. This situation might be expected if the metal–hydrogen species is predicted to act as an intermediate in a catalytic process. Under these con- ditions, the low concentration of the metal–hydrogen species would render the NMR approach less effective as a tool for characterisation, due to the inherent insensitivity of the NMR technique. While kinetic evidence plays in important role in determining the mechanisms of such catalytic processes, and theoretical methods may predict the existence and structure of the associated reaction intermediates, the detection and charac- terisation of these intermediate species remains essential. For this reason, any method by which the sensitivity of NMR spec- troscopy can be increased is potentially significant. One such method is the use of parahydrogen (p-H 2 ), molecular hydrogen 0010-8545/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2008.01.028