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 fr equencies, is a useful tool for investigating proximal ligand and distal en vironmental effects in CO adducts of heme proteins and models. The data fal l on parallel lines of negative slope, shifted along the Fe-CO axis for pro ximal ligands of different donor strength. However, new resonance Raman spe ctra 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-l ocal density functional theory on Fe(II)(porphines)(CO) with fluorine, chlo rine and amino substituents. The calculations likewise give lines of negati ve slope, displaced according to trans ligand donor strength. Strikingly, t he slopes are nearly the same for pyridine and methylthiolate trans ligands , but halved for five-coordinate complexes, as in the experimental plots. H owever, the magnitude of the slopes, as well as the displacements of the co rrelations, are about twice the experimental values. The origin of the back bonding behavior can be seen in the bond-length relationships for the calcu lated structures. For a given C-O length, the Fe-CO bond is lengthened syst ematically by increasing the donor strength of the axial ligand, whose sigm a 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 sl opes increase systematically with increasing ligand donor strength, because the longer Fe-CO distance decreases the pi overlap, and increases the Fe-C O sensitivity to backbonding changes. (C) 2000 Elsevier Science S.A. All ri ghts reserved.