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Why is it that cyanide is an inhibitor of catalase but carbon monoxide isn't? What makes cyanide bind well to $\ce{Fe^3+}$?

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Jan has already covered this topic in a related question: Why is the cyanide ion toxic?

$\ce{CN-}$ and $\ce{CO}$ are isoelectronic, and one might expect them to behave exactly similarly. However, there is a key difference: the orbital energies in $\ce{CN-}$ are higher than in $\ce{CO}$, due to the lower electronegativity of $\ce{N}$ compared to $\ce{O}$.

This means that $\ce{CN-}$ is a much better σ donor, and a much poorer π acceptor than $\ce{CO}$. Consequently $\ce{CN-}$ is much happier binding to a metal in a higher oxidation state, whereas $\ce{CO}$ is much happier binding to a metal in a lower oxidation state.

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