In the absence of bound globin chain, heme group on exposure to $\mathrm{O_2}$ gives the iron-oxygen species
(a)
(b)
(c)
(d)$\mathrm{Fe(IV)=O}$
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
Without the globin pocket, the Fe(II)–O₂ adduct reacts with a second heme to form a peroxo-bridged dimer that collapses irreversibly to the μ-oxo Fe(III)–O–Fe(III) dimer (a).