The reaction 2 NO₂ → 2 NO + O₂ is proposed to occur by the elementary steps
(1) NO₂ + NO₂ ⇌ N₂O₄ (fast, reversible)
(2) N₂O₄ → 2 NO + O₂ (slow)
Which species formed in step (1) is an intermediate that would not appear in the rate law for the overall reaction?