Four new bivalent nickel hydrazone complexes have been synthesised from the reactions of [NiCl₂(PPh₃)₂] with H₂L {L = dianion of the hydrazones derived from the condensation of salicylaldehyde or o-hydroxy acetophenone with p-toluic acid hydrazide (H₂L¹) (1), (H₂L²) (2) and o-hydroxy acetophenone or o-hydroxy naphthaldehyde with benzhydrazide (H₂L³) (3) and (H₂L⁴) (4)} and formulated as [Ni(L¹)(PPh₃)] (5), [Ni(L²)(PPh₃)] (6), [Ni(L³)(PPh₃)] (7) and [Ni(L⁴)(PPh₃)] (8). Structural characterization of complexes 5–8 were accomplished by using various physico-chemical techniques. In order to study the influence of substitution in the ligand and its planarity on the biological activity of complexes 5–8 containing them, suitable hydrazone ligands 1–4 have been selected in this study. Single crystal diffraction data of complexes 5, 7 and 8 proved the geometry of the complexes to be distorted square planar with a 1 : 1 ratio between the metal ion and the coordinated hydrazones. To provide more insight on the mode of action of complexes 5–8 under biological conditions, additional experiments involving their interaction with calf thymus DNA (CT DNA) and bovine serum albumin (BSA) were monitored by UV-visible and fluorescence titrations respectively. Further, the ligands 1–4 and corresponding nickel(II) chelates 5–8 have been tested for their scavenging effect towards OH and O₂⁻ radicals. The effect of complexes 5–8 to arrest the growth of HeLa and Hep-2 tumour cell lines has been studied along with the cell viability against the non-cancerous NIH 3T3 cells under in vitro conditions.