Unexpected explanation for the enigmatic acid-catalysed reactivity of [Fe₄S₄X₄]²⁻ clusters

Density functional calculations show that Fe–S clusters undergo unexpected large structural changes when protonated at S. Protonation of prototypical cubanoid [Fe₄S₄X₄]²⁻ to [Fe₄S₃(SH)X₄]⁻ (X = Cl, SR, OR) results in formation of doubly-bridging SH, severance of one Fe–S bond, and creation of a three-coordinate Fe. These findings explain previously enigmatic results concerning the reactivity of these clusters, including the rates of protonation, pK_a data, and the kinetics of acid-catalysed ligand substitution.