To shed more light onto the hydrogen transfer mechanism, a QTAIM analysis 60,61 of 2 and TS1 was performed. QTAIM analysis of complex 2 (Figure , b) showed bond critical points (BCP) from both carbons of the olefin and nickel located in the ionic area and the BCP of the olefin in the covalent area (C41-Ni1 Ñr 2 = 0.250, C10-Ni1 Ñr 2 = 0.237 and C41-C10 Ñr 2 = -0.753), as typical for metal olefin complexes. The interaction with nickel and H2 shows a bond path that originates from the nickel atom towards the center of the H-H bond, but curves towards one of the hydrogen atoms (Ni1-H95 Ñr 2 = 0.395). This can be explained as a "bond catastrophe": the BCP in between Ni-H96 and the RCP for Ni-H95-H96 have coalesced, as has been observed before in the analysis of other non-classical dihydrogen complexes and olefins. QTAIM analysis of TS1 present interesting features (Figure ). The Ni-bound olefinic backbone displays similar BCP characteristic values to complex 2 (C41-Ni1 Ñr 2 = 0.259 and C10-Ni1 Ñr 2 = 0.242 and C41-C10 Ñr 2 = -0.707). The H-H bond is broken, but both atoms are bonded to nickel. Both interactions with nickel are ionic with a slight difference between them (Ni-H95 Ñr 2 = 0.121 and Ni-H96 Ñr 2 =0.084) but a significant increase in covalency with respect to the Ni-H2 interaction in complex 2. A BCP between C41 and H95 located in the ionic area (Ñr 2 = 0.010) corresponds to the barely formed C-H bond. These observations support the description of TS1 as resembling an olefin-bound Ni(II) dihydride (NiH2) complex. This description of TS1 is consistent with a ligand-to-ligand hydrogen transfer mechanism (LLHT) consisting of a concerted hydrogen atom transfer between two ligands (the H2 molecule and the olefin) with concomitant oxidation of nickel. This mechanism was first described for the Ni-catalyzed hydrofluoroarylation of alkynes: a fluoroarene first coordinates in h 2 (C,H) fashion to an h 2 (C,C)-alkyne nickel(0) complex to then transfer an H atom to the alkyne without a hydride intermediate, generating an agostic vinyl group. This mechanism is favored for metals with a small radius such as nickel and LLHT steps have been invoked for C-H bond activation reactions at nickel/olefin complexes. . Alternatively, TS1 could be viewed as a s-CAM type mechanism if the olefin adduct is described as the nickelacyclopropane extreme of the DCD model. In that case the oxidation state of Ni(II) does not change and the transfer occurs between s-bonds. Catalytic activity in the semihydrogenation of alkynes. Having found that complex 1 activate H2 efficiently and reversibly, we investigated the applicability of H2 activation by LLHT in catalytic hydrogenation. Inspired by recent progress in the nickel-catalyzed semihydrogenation of internal alkynes using H2 as hydrogen source, Reaction conditions: J-Young NMR tube, precatalyst = complex 1, 70 ºC oil bath, volume 0.55 mL of d 8 -tol, conversion and products were monitored by 1 H-NMR using mesitylene as internal standard.