Ruthenium and Osmium form RuO4 and OsO4 which are in (+8) oxidation state. Ru(iii) and Os (+iv) are the most stable State. Ru(+v) , Os(vi) and Os ( viii) are also stable.
Thus , a trend is observed that , on descending a group, the higher oxidation state become more stable . Therefore RuO4 and OsO4 are very stable with an oxidation state of +8 . Iron being the first in the group is not able to form a stable iron oxide with oxidation state of 8. Ruthenium and Osmium have no oxides compared to those of iron and indeed, the lowest oxidation state in which they form oxides is +4 .
