Three mononuclear complexes [Co(biq)X2] (biq = 2,2'-biquinoline; X = Cl, Br, I) were prepared by a solvothermal method and characterized by single-crystal X-ray diffraction. In all three complexes the Co(ii) atom is tetrahedrally coordinated by one biq ligand bonded in a chelate manner and two halogenido ligands. Hydrogen bonding interactions (C-HX) along with the π-π interactions contribute to the stability of the formed packing. Magnetic measurements as well as ab initio calculations revealed that the complexes possess a sizable easy-plane magnetic anisotropy (D > 0). They display a superparamagnetic behaviour in an applied external field that culminates between BDC = 0.2-0.3 T. Two relaxation processes are observed; the faster can be analysed in terms of the direct and Orbach processes yielding U/kB = 42.6 K and τ0 = 1.9 × 10(-10) s for X = Cl, U/kB = 39.6 K and τ0 = 1.2 × 10(-10) s for X = Br and U/kB = 57.0 K and τ0 = 3.2 × 10(-13) s for X = I at BDC = 0.2 T.