Why Iron shows maximum of +6 oxidation state while in the same group ruthenium and osmium shows +8 as maximum oxidation state?

it just must be that the 4s electrons are split by more energy from the 3d electrons than the 4d and 5s electrons are in the analogous atoms further down, so that the energy gap is sufficient to grab an electron from another atom in the environment.
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Mukesh Sharma
AskIITians Faculty