Explain why iron, cobalt, nickel do not show the expected highest oxidation states of +8, +9, and +10 resp?

Co forms co+2 due to the loss of two electrons from 4s subshell and by further Loss of one more electron from 3d orbital forms co3+. In cobalt +2 & +3 oxidation are more stable due to the exchange energy of 3d6 electrons and does not lose electrons further. Hence co does not show +9 oxidation state.