Density Functional Studies on 19-Electron Metal Sandwich Complexes:  Electronic Structures of CpFe(η⁶-C₆H₆), CpFe(η⁶-C₆Me₆), and (C₅Me₅)Fe(η⁶-C₆H₆)

Abstract

Density functional theory (DFT) calculations were carried out on three 19-electron metal sandwich complexes, CpFe(η 6 -C 6 H 6 ), CpFe(η 6 -C 6 Me 6 ), and (C 5 Me 5 )Fe(η 6 -C 6 H 6 ), to determine their electronic structure. The nuclear quadrupole splittings for these three molecules were also calculated using DFT and were found to be in reasonable agreement with the experimental values observed by Mo ¨ssbauer spectroscopy. For CpFe(η 6 -C 6 H 6 ) and (C 5 Me 5 )-Fe(η 6 -C 6 H 6 ), the metal contributions to the singly occupied molecular orbital (SOMO) were found to be about 45% and 30%, respectively, lower than estimates based on the magnitude of the quadrupole splitting. In the case of CpFe(η 6 -C 6 Me 6 ), however, the SOMO is dominated by the permethylated arene ligand, while the metal contribution is only about 10%, much less than the estimate of 75% based on the quadrupole splitting. In all three molecules the spin density is concentrated around the iron atom, and the partial charge on the iron atom is 0.75, as calculated using the AIM method. It is concluded that the determination of the composition of a molecular orbital based on comparing the observed quadrupole splitting for a molecule to the estimated splitting for the free atom is too approximate to be reliable and that consideration of a single molecular orbital does not allow a reliable estimate of the net charge or spin density around an atom in a molecule.

Investigation of organometallic reaction mechanisms with one and two dimensional vibrational spectroscopy

“…The relatively small changes indicate that the transition from a 17e to 19e radical does not appreciably affect the electron density surrounding the metal atom. This result agrees well with previous DFT studies on transition-metal radicals; a collective change in the electron distributions of numerous molecular orbitals helps to preserve the net electron density on the metal [132,170,207,208]. Nevertheless, the '19th', unpaired electron is localized on the iron atom; thus, we consider the complexes are appropriately labeled 19e, metal-centered radicals and not 18+δ compounds.…”

“…These types of calculations have been shown reliable in comparison to experimental data on 19e complexes [132,170,207,208]. The results are tabulated in Table 5.3 and show little variation between the four radicals R 1 -R 4 .…”

“…All the radical complexes also exhibit a relatively large degree of spin polarization, which causes an appreciable amount of β spin density on various fragments of the complexes in addition to the α spin density associated with the unpaired electron [186,187]. Spin polarizations effects have been reported previously for these types of complexes [132,170,207,208].…”

“…The observed and calculated vibrational frequencies are summarized in Table 5.2, and the molecular structures for the radicals are depicted in Figure 5.11. DFT calculations have been shown reliable for predicting the energetics, vibrational frequencies, electronic structure, and molecular geometries of organometallic radicals [21][22][23][24]132,170,[207][208][209]. Figure 5.11a shows the 17e radical CpFe(CO) 2 (R 1 ) which serves as the reference point for the reported energies of all other radical species.…”

Fluxional behavior of methyl-substituted tricarbonyl(tropone)iron complexes and their different reactivity

“…The selected functionals, especially B3P86, turned out to be adequate for optimization, of the ground state geometry of organometallic compounds [30]. In many cases the DFT approach leads to geometries comparable with the MP2 geometries if not even closer to the experimental ones [31].…”