Abstract: Nickel(II) complex, [NiII(L)2] of the ligand 1,3-dimethyl-5-(p-Cl-phenylazo)-6-aminouracil (HL) was isolated in the form of single crystals and characterized by single-crystal X-ray diffraction. The complex possesses square planar geometry with molecular formula C24H22Cl2N10NiO4. The atomic arrangements about the Ni(II) are in trans-square planar fashion where the Ni(II) ion lies on an inversion centre. It crystallizes in the orthorhombic system, space group Pbca, with lattice parameters: a = 11.9282(7) Å, b = 11.2222(6) Å, c = 19.9242(12) Å and α = β = γ = 90°. The complex is stable in neutral to basic buffer solution region and dissociates to its anionic hydrazone form followed by the hydrazone in acidic buffer solution. The dissociation process was studied kinetically and is described as “acid-limiting” or “saturation kinetics”, in which the rate becomes independent of the hydrogen ion concentration at high acid concentration. The process involves rapid protonation pre-equilibrium (constant, K) followed by rate-determining dissociation (constant, k) step and these constants are found to be 3.41 × 10−6 M−1 and 8.37 × 10− M−1s−1, respectively. The interaction of the complex with different anions like F−, Cl−, Br−, AcO−, NO3 − or HSO4 − was studied and found quite different results. With fluoride ions the complex using its –NH- fragments, forms H-bonded association complex whereas in presence of HSO4 − the complex like in acidic buffer solution, dissociates to form (kd = 1.28 M−1s−1) the hydrazone form of the ligand. Graphical Abstract: The square planar nickel(II) complex of the ligand 1,3-dimethyl-5-(p-Cl-phenylazo)-6-aminouracil forms association complex with F− ions and dissociates to form the hydrazone form of the ligand in acidic buffer solution and in presence of HSO4 −.[Figure not available: see fulltext.]. © 2017, Springer Science+Business Media, LLC.