Question
Why does the position of the V=O stretch in the IR spectrum change as other ligands are added trans to the V=O bond? What does this tell you about the electron donating properties of the N-donor ligands and their relative strengths?
Answer
In studying the effect on the V=O stretch of adding other ligands trans to the V=O bond, acetyl acetone (acac) is often used as a ligand. This is a bidentate ligand which forms a complex VO(acac)2 (shown in Figure 1) in which two weakly aromatic rings are formed, effectively blocking the positions cis to the V=O bond from further substitution.
Figure 1: VO(acac)2 (L) and VO(acac)2Py (R)
Addition of pyridine (Py) trans to the V=O bond, to form VO(acac)2Py (also shown in Figure 1) causes a change in the frequency of the V=O stretch from 997 cm-1 in VO(acac)2 to 965.3 cm-1 in VO(acac)2Py (Nyquist & Kagel, 1996). This is due to the co-ordinate bond formed between the lone pair of electrons on the nitrogen and the empty d-orbital of the vanadium. This increases the electron density around the vanadium, resulting in repulsion of the V=O bond. This repulsion causes the V=O bond to lengthen and therefore weaken, resulting in a lower wavenumber for the V=O stretch. Other nitrogen donor ligands, such as piperidine, have a similar effect in reducing the wavenumber; however, in the case of piperidine, the reduction is lower (Nyquist & Kagel, 1996). This is because pyridine has a higher electron density than piperidine, due to the aromatic ring; it therefore causes a greater increase in the electron density around the vanadium and hence a greater weakening of the V=O bond (Suzuki & Orvilelle-Thomas, 1977). Addition of an oxygen-donor ligand in the trans position also causes a reduction in the V=O wavenumber; however, this is much smaller, in the region of 5-10 cm-1 (Larsson, 1972), demonstrating that oxygen is a weaker electron donor than nitrogen.
References
Larsson, R., 1972. Solvent effectson the infrared intensity of the V=O stretching vibration in vanadyl acetoacetonate. Acta Chimica Scandinavia, Volume 26, pp. 549-555.
Nyquist, R. & Kagel, R., 1996. Handbook of infrared and raman spectra of inorganic compounds and organic salts: infrared spectra of inorganic compounds. Cambridge, MA: Academic Press.
Suzuki, S. & Orvilelle-Thomas, W., 1977. Molecular force field of pyridine and its application to pyridine-metal complexes. Journal of Molecular Structure, 37(2), pp. 321-327.
Pietro, W., & Lever, A., 2022. Ligand Electrochemical Parametre Approach to Molecular Design. σ-Donation, π-Back Donation, and Other Metrics in Ruthenium(II) Dinitrogen Complexes.. Inorganic chemistry. https://doi.org/10.1021/acs.inorgchem.1c02707.
Smith, J., Logan, J., Doyle, L., Burford, R., Sugawara, S., Ohnita, C., Yamamoto, Y., Piers, W., Spasyuk, D., & Borau-Garcia, J., 2016. Cationic mono and dicarbonyl pincer complexes of rhodium and iridium to assess the donor properties of PCcarbeneP ligands.. Dalton transactions, 45 32, pp. 12669-79 . https://doi.org/10.1039/c6dt02615j.
Chakraborty, S., Patzer, A., & Dopfer, O., 2010. IR spectra of protonated benzaldehyde clusters, C(7)H(7)O(+)-L(n) (L=Ar,N(2);nThe Journal of chemical physics, 133 4, pp. 044307 . https://doi.org/10.1063/1.3460458.
Mishra, S., Nguyen, H., Huang, Q., Lin, C., Kuo, J., & Patwari, G., 2020. Vibrational spectroscopic signatures of hydrogen bond induced NH stretch-bend Fermi-resonance in amines: The methylamine clusters and other N-H⋯N hydrogen-bonded complexes.. The Journal of chemical physics, 153 19, pp. 194301 . https://doi.org/10.1063/5.0025778.
Berto, T., Praneeth, V., Goodrich, L., & Lehnert, N., 2009. Iron-porphyrin NO complexes with covalently attached N-donor ligands: formation of a stable six-coordinate species in solution.. Journal of the American Chemical Society, 131 47, pp. 17116-26 . https://doi.org/10.1021/ja904368n.
Tatsumi, T., Tominaga, H., Hidai, M., & Uchida, Y., 1980. Preparation and properties of dinitrogenmolybdenum complexes. Journal of Organometallic Chemistry, 199, pp. 63-75. https://doi.org/10.1016/S0022-328X(00)84523-4.
Gusev, D., 2009. Donor Properties of a Series of Two-Electron Ligands. Organometallics, 28, pp. 763-770. https://doi.org/10.1021/OM800933X.
Groenewold, G., Gianotto, A., Cossel, K., Van Stipdonk, M., Moore, D., Polfer, N., Oomens, J., De Jong, W., & Visscher, L., 2006. Vibrational spectroscopy of mass-selected [UO2(ligand)n]2+ complexes in the gas phase: comparison with theory.. Journal of the American Chemical Society, 128 14, pp. 4802-13 . https://doi.org/10.1021/JA058106N.
