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We report results, based on density functional theory− generalized gradient approximation calculations, that shed light on how NO, CO, and O 2 interact with Fe 2 S 2 , Fe 3 S 4 , and Fe 4 S 4 clusters and how they modify their structural and electronic properties. The interest in these small iron sulfide clusters comes from the fact that they are at the protein cores and that elucidating fundamental aspects of their interaction with those light molecules which are known to modify their functionality may help in understanding complex behaviors in biological systems. CO and NO are found to bind molecularly, leading to moderate relaxations in the clusters, but nevertheless to changes in the spin-polarized electronic structure and related properties. In contrast, dissociative chemisorption of O 2 is much more stable than molecular adsorption, giving rise to significant structural distortions, particularly in Fe 4 S 4 that splits into two Fe 2 S 2 subclusters. As a consequence, oxygen tends to strongly reduce the spin polarization in Fe and to weaken the Fe−Fe interaction inducing antiparallel couplings that, in the case of Fe 4 S 4 , clearly arise from indirect Fe−Fe exchange coupling mediated by O. The three molecules (particularly CO) enhance the stability of the iron−sulfur clusters. This increase is noticeably more pronounced for Fe 2 S 2 than for the other iron−sulfur clusters of different compositions, a result that correlates with the fact that in recent experiments of CO reaction with Fe m S m (m = 1−4), the Fe 2 S 2 CO product results as a prominent one.

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