Density functional theory investigation of the active site of Fe-hydrogenases. Systematic study of the effects of redox state and ligands hardness on structural and electronic properties of complexes related to the [2Fe](H) subcluster

Density functional theory has been used to investigate complexes related to the [2Fe](H) subduster of [Fe]-hydrogenases. In particular, the effects on structural and electronic properties of redox state and ligands with different sigma-donor T-acceptor character, which replace the cysteine residue coordinated to the [2Fe](H) subduster in the enzyme, have been investigated. Results show that the structural and electronic properties of fully reduced (FeFeI)-Fe-I complexes are strongly affected by the nature of the ligand L, and in particular, a progressive rotation of the Fe-d(CO)(2)(CN) group, with a CO ligand moving from a terminal to a semibridged position, is observed going from the softest to the hardest ligand. For the partially oxidized (FeFeII)-Fe-I complexes, two isomers of similar stability, characterized either by a CO ligand in a terminal or bridged position, have been observed. The switching between the two forms is associated with a spin and charge transfer between the two iron atoms, a feature that could be relevant in the catalytic mechanism of dihydrogen activation. The structure of the fully oxidized (FeFeII)-Fe-II models is extremely dependent on the nature of the L ligand; one CO group coordinated to Fed switches from terminal to bridging position going from complexes characterized by neutral to anionic L ligands