There may be good chemical reasons for this. Complexes of first row transition metals tend to be less kinetically stable than those of the second row and especially the third row of transition metals. Also, essential transition (d-block) metals (Mn, Fe, Co, Cu, Zn, and Mo) have metabolic pathways specifically designed to control their homeostasis (uptake, transport, storage and usage), so they may become diverted into targets other than the desired ones in the infective organisms.