Density Functional Studies of 19-Electron Organometallic Complexes:  Investigation of Possible Ligand Distortions and Calculation of the EPR Parameters and Unpaired Electron Distributions in CpCr(CO)₂NO^-, CpW(NO)₂P(OMe)₃, CpMo(CO₃)P(OMe)₃, and Co(CO)₃(2,3-bis(diphenylphosphino)maleic anhydride)

Abstract

Density functional theory (DFT) was used to calculate the equilibrium geometries and EPR hyperfine coupling constants for the CpCr(CO) 2 NOand CpW(NO) 2 P(OMe) 3 19-electron complexes. The calculated EPR hyperfine parameters for both molecules were found to be in good agreement with the experimental values. The EPR study on CpCr(CO) 2 NOindicated that the nitrosyl ligand bends significantly when the neutral 18-electron parent molecule is reduced. This was confirmed by the DFT calculations, although the degree of bending is not as large as that predicted from the EPR analysis. The calculated geometry for CpW(NO) 2 P-(OMe) 3 showed that the nitrosyl ligands are not significantly bent, which is consistent with the crystal structure of the related compound CpW(NO) 2 P(OPh) 3 . In both CpCr(CO) 2 NOand CpW(NO) 2 P(OMe) 3 , the unpaired electron is localized to about the same extent on the NO ligands. It is concluded that, although distortion of a ligand implies significant localization of the unpaired electron on that ligand, an undistorted ligand does not imply that the unpaired electron has no appreciable amplitude on that ligand. DFT was also applied to the 19-electron CpMo(CO) 3 P(OMe) 3 molecule, whose role as a key intermediate in photochemical reactions of Cp 2 Mo 2 (CO) 6 with P(OMe) 3 has been inferred from mechanistic studies. It was found that loss of P(OMe) 3 from CpMo(CO) 3 P(OMe) 3 is slightly thermodynamically favorable (∆E ) ∼1 kcal/mol). This is consistent with the necessity of using an excess of the phosphorus ligand in reactions in which CpMo(CO) 3 PR 3 is believed to be the electron-transfer agent. There is no indication of P(OMe) 3 ligand distortion in this molecule, just as there is no indication of Cp ring slippage in any of the Cp-containing molecules. Finally, a calculation of the hyperfine coupling constants in the 18 + δ Co(CO) 3 L 2 (L 2 ) 2,3-bis(diphenylphosphino)maleic anhydride) complex was also carried out. The calculated values are in reasonable agreement with experiment.

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.…”

NO binds to unsaturated Os(IV) polyhydrides as a redox reagent

“…NO is a ligand which is highly redox active, including a recently recognized neutral form [1][2][3][4][5]. Even electron species are inherent to the 18-electron rule, as are two electron processes during reaction.…”