A series of mononuclear tetrahedral CoII complexes with a general molecular formula [CoL2X2] [L=thiourea and X=Cl (1), Br (2) and I (3)] were synthesized and their structures were characterized by single-crystal X-ray diffraction. Direct-current (dc) magnetic susceptibility [χMT(T) and M(H)] and its slow relaxation of magnetization were measured for all three complexes. The experimental dc magnetic data are excellently reproduced by fitting both χMT(T) and M(H) simultaneously with the parameters D=+10.8 cm−1, g1=2.2, g2=2.2, and g3=2.4 for 1; D=−18.7 cm−1, giso=2.21 for 2; and D=−19.3 cm−1, giso=2.3 for 3. The replacement of chloride in 1 by bromide or iodide (in 2 and 3, respectively) was accompanied by a change in both sign and magnitude of the magnetic anisotropy D. Field-induced out-of-phase susceptibility signals observed in 10 % diluted samples of 1–3 imply slow relaxation of magnetization of molecular origin. To better understand the magnetization relaxation dynamics of complexes 1–3, detailed ab initio CASSCF/NEVPT2 calculations were performed. The computed spin Hamiltonian parameters are in good agreement with experimental data. In particular, the calculations unveil the role of halide ions in switching the sign of D on moving from Cl− to I−. The large spin–orbit coupling constant associated with the heavier halide ion and weaker π donation reduces the ground state–excited state gap, which leads to a larger contribution to negative D for complex 3 compared to complex 1. Further magnetostructural D correlations were developed to understand the role of structural distortion in the sign and magnitude of D values in this family of complexes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim