Abnormal N‐heterocyclic carbene (aNHC) based Ni(II) π‐allyl complexes (3 and 4) were synthesized starting from a Ni(0) precursor. These complexes were characterized by NMR spectroscopy, single‐crystal X‐ray crystallography (4) and elemental analysis data. The underlying mechanism for the formation of Ni(II) η3‐allyl complexes from a Ni(0) precursor on treatment with a free abnormal N‐heterocyclic carbene in absence of any external additive or oxidant was unraveled. Later, complex 3 was exposed to O2 gas under ambient pressure resulting in molecular oxygen activation to form a μ‐hydroxo bridged Ni(II) dimer.