$$ \text{O} + n_1\text{e}^- \rightleftharpoons \text{R} $$
$$ \text{R} \mathrel{\underset{k_{b1}}{\overset{k_{f1}}{\rightleftharpoons}}} \text{X} $$
$$ \text{X} + n_2\text{e}^- \rightleftharpoons \text{Y} $$
$$ \text{Y} \mathrel{\underset{k_{b2}}{\overset{k_{f2}}{\rightleftharpoons}}} \text{Z} $$