Computational definition of a mixed valent Fe(II)Fe(I) model of the [FeFe]hydrogenase active site resting state

Density-functional calculations have been used to examine the electronic structure and bonding in the recently reported complex [(PMe3)(CO)(2)Fe(mu-pdt)(mu-CO)Fe(CO)(IMes)](+) (1(+), IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene). This mixed valent (FeFeI)-Fe-II complex features a rotated geometry that places a carbonyl ligand in a semi-bridging position, which makes it an accurate model of the S = 1/2 resting state of the [FeFe]-hydrogenase active site. Calculations indicate that the unpaired electron in this complex lies almost entirely on the rotated iron center, implying that this iron remains in the Fe-I oxidation state, while the unrotated iron has been oxidized to Fe-II. The frontier molecular orbitals in 1(+) are compared with those in the neutral (FeFeI)-Fe-I precursor (PMe3)(CO)(2)Fe(mu-pdt)(mu-CO)Fe(CO)(IMes) at both its optimized geometry (1) and constrained to a rotated geometry (1(rot)). These theoretical results are used to address the role of the bridging CO ligand in 1(+) and to predict reactivity patterns; they are related back to the intricate biological mechanism of [FeFe]-hydrogenase. (c) 2007 Elsevier Inc. All rights reserved.