Unlike Cr3+ , Mn 2+ , Fe 3+ and the subsequent other M2+ ions of the 3d series of elements , the 4d and 5d series metals generally do not form stable cationic species. Why?

hey student..
it is not that the 4d and 5d series elements do not form stable cationic species. they do form such species but the only thing here is the possibility of other (variable) oxidation states and this is due to the poor shielding of d and f electrons