A trend of increasingly more negative zero-field splitting parameters D was observed for a series of pseudo-tetrahedral mononuclear cobalt complexes, Co(L)2I2 (1–3) (L = quinolone, PPh3 or AsPh3) in going down the group from N to P and As (1 = +9.2 cm−1, 2 = −36.9 cm−1 and 3 = −74.7 cm−1) with 2 and 3 exhibiting slow relaxation of the magnetization under an applied DC field.