The effect of ligands with heavy donor atoms on the magnetic anisotropy of the pseudo-tetrahedral cobalt complexes, Co(quinoline)₂I₂ (1) and Co(EPh₃)₂I₂ (2–3) (E = P, As) has been investigated. The axial zero-field splitting parameter D was found to vary from +9.2 cm⁻¹ in 1 to −36.9 cm⁻¹ in 2 and −74.7 cm⁻¹ in 3. Compounds 2 and 3 exhibit slow relaxation of the magnetization up to 4 K under an applied dc field, indicating SMM behavior.