No abstract

Electronic absorption, Fe K-edge X-ray absorption, resonance Raman, and Mössbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 Fe IV =O unit, namely i) the near-IR absorption properties, ii) X-ray absorption pre-edge intensities, and iii) quadrupole splitting parameters, are strongly dependent on the identity of the trans ligand. However, on the basis of EXAFS data, most [Fe IV (O)(TMC)(X)] + species have Fe=O bond lengths similar to that of [Fe IV (O)(TMC)(NCMe)] 2+ (1.66 ± 0.02 Å). The mechanisms by which the trans ligands perturb the Fe IV =O unit were probed using density functional theory (DFT) computations, yielding geometric and electronic structures in good agreement with our experimental data. These calculations revealed that the trans ligands modulate the energies of the Fe=O σ-and π-antibonding molecular orbitals, causing the observed spectroscopic changes. Time-dependent DFT methods were used to aid in the assignment of the intense near-UV absorption bands found for the oxoiron(IV) complexes with trans N 3 − , NCS − , and NCO − ligands as X − -to-Fe IV =O charge transfer transitions, thereby rationalizing the resonance enhancement of the ν(Fe=O) mode upon excitation of these chromophores.