Role of the axial ligand in heme-CO backbonding; DFT analysis of vibrational data

The FeCO backbonding correlation, a plot of C-O versus Fe-CO vibrational frequencies, is a useful tool for investigating proximal ligand and distal environmental effects in CO adducts of heme proteins and models. The data fall on parallel lines of negative slope, shifted along the Fe-CO axis for proximal ligands of different donor strength. However, new resonance Raman spectra of five-coordinate Fe(II)(porphyrins)(CO) with electron-withdrawing or -donating substituents reveal that the backbonding slope is reduced in the absence of a proximal ligand. This phenomenon has been explored with non-local density functional theory on Fe(II)(porphines)(CO) with fluorine, chlorine and amino substituents. The calculations likewise give lines of negative slope, displaced according to trans ligand donor strength. Strikingly, the slopes are nearly the same for pyridine and methylthiolate trans ligands, but halved for five-coordinate complexes, as in the experimental plots. However, the magnitude of the slopes, as well as the displacements of the correlations, are about twice the experimental values. The origin of the backbonding behavior can be seen in the bond-length relationships for the calculated structures. For a given C-O length, the Fe-CO bond is lengthened systematically by increasing the donor strength of the axial ligand, whose sigma donor orbital competes with the CO donor sigma orbital for the vacant Fe d(z²) orbital. For a given proximal ligand, the Fe-CO and C-O bond lengths are negatively correlated, as expected for backbonding. In addition, the slopes increase systematically with increasing ligand donor strength, because the longer Fe-CO distance decreases the π overlap, and increases the Fe-CO sensitivity to backbonding changes. (C) 2000 Elsevier Science S.A.