The calculated binding energy of 1a to {Ni(CO)Cp} to form 2 is only mildly exergonic (ΔG o 298K = -0.5 kcal mol -1 ) and coordination might be expected to be reversible in solution. Solution-based magnetic susceptibility measurements on 2 obtained via the Evans method reveal an effective magnetic moment of µeff = 1.8 µB, slightly beyond that predicted by the spin-only formula µso = 1.7 µB for an S = ½ complex. Complex 2 was also characterized by electron paramagnetic resonance (EPR) spectroscopic methods. The room-temperature continuous wave (CW) EPR spectrum of a solution of 2 in toluene exhibits a single resonance at giso = 2.0524 (Figure ) that splits into an axial signal upon formation of a frozen solution at 80 K (Figure ) with g values g‖ = 2.1020 and g ⊥ = 2.0204. These g values are indicative of a Ni(I) d 9 centre with an S = ½ ground state, rather than a Ni(III) d centre which typically exhibit much larger average g values due to ligand field and symmetry effects. While these data could be interpreted based on a retention of the structure of 2 in solution, typically transition metal hydrides produce a relatively large and anisotropic hyperfine coupling. In conflict with this expectation, a hyperfine coupling of the unpaired electron to the bridging hydride of 2 was not observed in CW EPR (Figure ). To probe whether such a coupling could have been masked in the CW EPR spectra, pulse EPR measurements were conducted. HYSCORE and Davies ENDOR (Figure ) measurements did uncover coupling to adjacent H nuclei, but the data are best explained by coupling to Cp ring protons, rather than the large near-axial hyperfine coupling of the bridging hydride predicted by DFT (Figures and, Tables ) and expected based on previous work. Based on these measurements, there is either insignificant electron spin density on the hydride of 2 or the EPR-active species in solution does not carry the bridging H atom. Hence 2 is at best dynamic in solution and may well be unstable with formation of a S = ½ nickel species that we cannot assign unambiguously (Figure ). Monitoring samples by EPR spectroscopy revealed a loss of intensity of the signal over time, consistent with the low stability of this system. Keen to understand whether the apparent dynamic binding of 1a could be leveraged in catalysis, a series of further reactions were undertaken. These experiments reveal a propensity for 1a (and an analogue) to participate in catalytic transformations that break and substitute the zinc hydride bond, which could be consistent with its reversible coordination and activation at a paramagnetic nickel fragment in solution.