NMR Data of Methyltitanium Trichloride and Related Organometallic Compounds. A Combined Experimental and Theoretical Study of Me_nXCl_4-n(n = 0-4; X = C, Si, Sn, Pb, Ti)

The complete series of Me_nTiCl_4-ncompounds has been prepared and the NMR spectra were recorded. The measured values are compared with the data of the main group equivalents Me_nXCl_4-nwith X = C, Si, Sn, and Pb. The ¹H, ^{1 3}C, ²⁹Si, and ^{47 49}Ti chemical shifts of the compounds Me_nXCl_4-n(X = C, Si, Ti) were calculated using the IGLO approach, based on optimized geometries at the HF and MP2 levels of theory. Theory and experiment agree that the central carbon atom of Me_nCCl_4-nshows a deshielding trend from n = 4 to 0. The central titanium atom in Me₄TiCl_4-nexhibits the opposite behavior, it becomes more shielded from n = 4 to 0. The calculated and experimental results show that the ¹³C and ^{47 49}Ti chemical shifts of MeTiCl₃are not anomalous, the data fit into the pattern observed and calculated for the whole series of Me_nTiCl_4-ncompounds. The silicon series Me_nSiCl_4-nexhibits a U-shaped curve. The analysis of the bond orbital contributions to the calculated shifts shows that the X-C and X-Cl localized bonds clearly dominate the theoretically predicted chemical shift of the atom X. The partitioning of the bond contributions into the diamagnetic and paramagnetic parts clearly demonstrates, that the paramagnetic contributions determine the trend of the chemical shifts of the three series of compounds. The diamagnetic parts are nearly constant. The paramagnetic contributions are particularly large for the Ti compounds, because Ti has low-lying empty d orbitals, which can interact with the bond orbitals by the action of the off-center angular momentum operator of Ti. The paramagnetic contributions are much lower for the Si and C compounds, because there are no low-lying empty orbitals available. The analysis of the chemical bonds using the NBO partitioning scheme shows that the polarization and spⁿhybridization of the C—CH₃, C—Cl, Si—CH₃, and Si—Cl bonds show the expected trends. The Ti—CH₃and Ti—Cl bonds are essentially sd³hybridized with negligible p contribution at Ti.