I believe similar questions have been asked but this is different and the answers I have looked at don't answer this question. I have read that oxidation states of heavier transition elements (Ru, Os etc) are more stable than first row transition metals.
I understand how they can get to the oxidation state (which is the usual answer to this question) but im asking why they are more stable then the oxidation state is reached. And although an element like Os is bigger meaning the ionisation energy should be small for the first few electrons doesn't the lanthanide contraction mean the valence electrons have a stronger effective nuclear attraction.
By more stable I mean why is RuO4 explosive but OsO4 on the other hand is stable and FeO4 doesn't exist.