  We report the existence of a sizeable quantum tunnelling splitting between
the two lowest electronic spin levels of mononuclear Ni complexes. The level
anti-crossing, or magnetic clock transition, associated with this gap has been
directly monitored by heat capacity experiments. The comparison of these
results with those obtained for a Co derivative, for which tunnelling is
forbidden by symmetry, shows that the clock transition leads to an effective
suppression of intermolecular spin-spin interactions. In addition, we show that
the quantum tunnelling splitting admits a chemical tuning via the modification
of the ligand shell that determines the crystal field and the magnetic
anisotropy. These properties are crucial to realize model spin qubits that
combine the necessary resilience against decoherence, a proper interfacing with
other qubits and with the control circuitry and the ability to initialize them
by cooling.
