Ti-backbonding

Ti-allylic substitution, with Pauson-Khand reaction, 11, 358 Ti-backbonding, organotransition metal CBC, 1, 29... 

Ti-backbonding from the metal to the ti antibonding orbital of the olefin weakens the C=C bond by up to 140 cm. ... 

IR spectral changes during the further reduction of Re(bpy)(CO)3Cl indicate rapid loss of a Cl ligand with formation of [Re(bpy)(CO)3] . The further reduction of either [Re(bpy)(CO)3(solvent)] or [Re(bpy)(CO)3]2 also yields [Re(bpy)(CO)3]. This illustrates the instability of non-Ti-backbonding ligands coordinated to the formally Re center. 

Infrared spectroscopy of olefin complexes is a less useful probe of n-bonding than infrared spectroscopy of CO complexes. Binding of an olefin to an electron-rich metal center does reduce the C-C stretching frequency, as one would expect from the reduction of the C-C bond order due to Ti-backbonding. However, the C-C stretch of a coordinated olefin is weaker than that of coordinated carbon monoxide because the vibration of the olefin creates a smaller change in the dipole moment. (Recall that symmetric vibrations are not observed in the infrared spectrum because of a lack of change in the dipole moment.) Thus, the olefin stretch is weak and lies at a frequency that overlaps with other bands. 

Fragalia and co-workers have reported the details of the He(I) and He(II) excited photoelectron spectra of Cp2Ti(CO)2 and concluded that evidence exists for significant backbonding between the Ti 3d orbitals and empty carbonyl v orbitals. Further, there is no evidence of important overlap between Ti and Cp orbitals. A small electrostatic perturbation of the Cp ligands is caused by the titanium atom (85). Bohm has described an elaborate study of the low energy PE spectrum of Cp2Ti(CO)2 (1) by means of semiempirical MO calculations (86). 

Complexes 41 and 42 were characterized by their IR and H-NMR spectra, and 41 also by elemental analysis. Table III contains the pertinent spectral data. Noteworthy are the very low energy terminal carbonyl bands for 41 and 42 at 1864 cm-1 (hexane). The weak 7r-accepting abilities of PR3 (R = Et, Ph) allow the lone CO ligand to 77-backbond to the Ti(II) center to a much greater degree. The -NMR spectrum of 41 exhibited a doublet (/H-p = 1.5 Hz) at 8 4.75 due to the coupling of the cyclo-pentadienyl protons with the 31P nucleus, while complex 42 exhibited a broad cyclopentadienyl singlet at 8 4.67. 

Incorporation of sterically demanding aryl substituents allows isolation of bis(arene)zirconium and hafnium complexes. The bond enthalpies of (7]6-(l,3,5-tBu)3C6H3)2M (M = Ti, 1 Zr, 2 Hf, 3) have been measured by iodinolytic bath calorimetry and values of 49(1), 64(3), and 67(4) kcal mol-1 have been determined for the respective metal-arene bond enthalpies (Scheme l).4 Computational studies establish that the major metal-arene bonding interaction is a 5-backbond formed from the overlap of metal dxz-yz and orbitals with the appropriate linear combination of arene p-orbitals. The observed increase in metal-arene bond strength is consistent with increased backbonding down the... 

Some of the most convincing evidence in support of pi backbonding comes from the IR spectra of the metal carbonyls. For the isolectronic series of compounds listed in Table 16.6, the v(CO) stretching frequency decreases as the electron density on the metal accumulates. The more electropositive the metal, the stronger the pi back-bonding, and the weaker the CO bond because of population of the ti CO) MO. 

Figure 21 is an energy diagram for a fully coordinated site. For reasons of simplicity the 4s and 4p orbitals are not taken into account, and the Ti—Cl bonds are assumed to be 100% ionic. The filled molecular orbitals include one 1 formed by ir bonding of olefin to metal ion and another < km formed by the stable orbital 2 is the result of backbonding of metal to olefin. 

Electrophilic attack on olefin ligands coordinated to electron-rich, strongly backbonding metals is illustrated by the reactions of (P group 4 olefin and alkyne complexes, as well as some electron-rich olefin complexes. Zirconocene- and and hafnocene-olefin complexes generated by reaction of zirconocene dichloride with two equivalents of alkyl lithium and isolated upon addition of a phosphine ligand react with carbonyl compounds and weak protic acids to form insertion products and alkyl complexes. Several examples of the reactions of these complexes with electrophiles are shown in Equations 12.65-12.66. Zirconocene-alkyne complexes prepared by thermolysis of vinyl alkyl complexes and titanium-alkyne complexes generated by the reduction of Ti(OPr ) also react with electrophiles, such as aldehydes and acid, to form products from insertion into the M-C bond and protonation of the M-C bond respectively. 

A well-developed series of complexes with rich MLCT excited-state behaviour are Re(I)-diimine complexes. [Re(bpy)(CO)3Cl] was the first transition metal complex used as a catalyst for CO2 reduction to CO, proposed by Lehn and Ziessel [41]. This series of complexes is particularly amenable to study of the excited state by time-resolved infrared spectroscopy. Formation of the MLCT Re bpy excited state leads to a reduction of the electron density on the metal centre. Consequently, d n backbonding from Re ti-orbitals to the antibonding n orbitals of CO ligands is reduced, resulting in an increase of the energy of the stretching vibrations, v(CO), by several tens of wavenumbers in the excited state if... 

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