Probing Hydrogen Activation in a Dimetal Dihydride Complex by Symmetric Exchange with Parahydrogen

We report an investigation into the hydrogen exchange mechanism in a dinickel(II) dihydride complex with C_2v symmetry. To obtain atomic level information on the low-concentration intermediate species, we use parahydrogen in combination with nuclear magnetic resonance (NMR). Unexpectedly, despite the chemical equivalence of the two hydride sites imposed by the mirror symmetry of the model, we observe spontaneous conversion of the nuclear singlet order into in-phase longitudinal magnetization-something that normally requires sophisticated pulse sequences. The magnetization is retained upon reductive elimination of dihydrogen from the dihydride species and observed as an enhanced hydrogen NMR signal. This is explained by a chain of nuclear relaxation interference processes that do not require asymmetric intermediates; we confirm this in two ways: using analytical relaxation theory and with brute-force numerical simulations.