Hydrogen activation. We had previously reported the synthesis of a Ni(0)-N2 complex of the bulky olefin pincer ligand Ph bppe H,CHptol2 by ring-opening of the corresponding nickelacyclobutane. In the solid state, the complex had been characterized as the N2-bridged dimer [( Ph bppe H,CHptol2 )Ni]2(µ-N2) (1 dimer , Figure ). In toluene solution under N2, an intense IR absorption at 2150 cm -1 indicates the presence of the mononuclear form ( Ph bppe H,CHptol2 )Ni(N2) (1, Figure ; see also SI section 2). The N2 coligand was easily displaced by addition of H2 (1 atm) to a solution of complex 1 in d 8 -tol. At 25 ºC, both 31 P{ 1 H} and 1 H NMR spectra displayed broad signals. Intriguingly, the expected signal of dissolved H2 was absent from the 1 H NMR spectra, suggesting exchange processes involving H2. Cooling the solution down to -40 ºC caused the reappearance of the H2 signal at 4.54 ppm and decoalescence of both 31 P{ 1 H} and 1 H NMR spectra to sharp lines, allowing for the identification of three Ni-containing species (Figure , a. See SI sections 1.1 and 1.2). Two minor species are observed: the first one is a residual amount of the known N2 complex 1 as shown by two 31 P NMR doublets at 11.3 and 28.2 ppm (JP,P= 53 Hz) and an 1 H NMR signal at d 4.75 ppm corresponding to the olefinic CH group. The second minor species displays similar characteristics: two 31 P NMR doublets at 15.5 and 32.8 ppm (JP,P= 58 Hz) and an olefinic 1 H signal at d= 4.35 ppm that was shown to couple with the 31 P NMR signal at 15.5 ppm by 1 H- P HMBC (see SI section 1.1). It also features a broad 1 H NMR signal at -2.1 ppm which suggested its assignment as the nonclassical-H2/olefin species 2. This was unambiguously confirmed by repeating the experiment under an HD atmosphere, which resulted in the appearance of a triplet signal (Figure , b) with a 1 JH,D coupling constant of 34 Hz, corresponding to a H-H bond length of 0.86 Å according to the Heinekey empirical relationship. The value corresponds to a true dihydrogen complex and is in good agreement with previous reports for Ni(0)-H2 complexes. The major species was assigned as the alkyl(hydrido) Ni(II) complex 3 resulting from cooperative H2 addition concomitantly forming new C-H and Ni-H bonds. A single, slightly broadened 31 P signal at 40.6 ppm (see SI 1.1) indicates two chemically equivalent P atoms. In the 1 H NMR spectrum, (Figure , b), a triplet signal at d -14.3 ppm ( 2 JH,P= 54 Hz) evidences a Ni-H bond with coupling to two equivalent phosphorus nuclei. A slightly broadened signal at d 3.1 ppm corresponds to the alkyl CH2 group, as also supported by a long-distance J-coupling with the Ni-H signal observed by 1 H-1 H COSY (see SI 1.1). The alkyl CH group appears as a triplet signal at d 3.8 ppm ( 3 JH,H= 8.4 Hz) due to coupling with the two equivalent hydrogen atoms of the CH2 group. Peak assignment was corroborated by an APT C experiment (see SI section 1.1). An 1 H-1 H EXSY experiment at -40ºC confirmed rapid chemical exchange between complex 2, complex 3 and dissolved H2 (Figure ). Namely, cross-peaks were observed between the Ni-H signal of 3 at -14.3 ppm and free H2 (4.54 ppm), the Ni-H2 signal of 2 at -2.1 ppm, and the CH2 signal of 3. In addition, exchange between free and Ni-bound H2 was also observed. These observations support facile and reversible transfer of a hydrogen atom from an Ni-bound H2 to an Ni-bound olefin with no detectable dihydride intermediate. DFT calculations support a concerted activation pathway (Figure , a). Exchange of N2 for H2 to form the non-classical Ni-H2 complex 2 from complex 1 is slightly endergonic (7.5 kcal/mol). A concerted transition state for H2 activation, TS1, was located 13.3 kcal/mol above complex 2 and is directly connected to structure 3a, a higher energy rotamer of the final alkyl(hydrido)nickel complex 3. The total exergonicity of -8.4 kcal/mol from 2 to 3 is consistent with 3 being the main species in solution but slightly overestimated. A putative nickel-dihydride structure was also located at 15.1 kcal/mol. However, forming this structure requires the olefin to leave the coordination sphere of Ni and no transition state connecting it to the products could be located.