Density functional theory calculations have been used to study the factors controlling the relative energies of the C- bound (2) and N-bound (3) tautomers of [Ru(Cl)(H)(CO)- (PPh3)2(IiPrMe2)] (IiPrMe2 = 3-isopropyl-4,5-dimethylimid- azol-2-ylidene) reported by Whittlesey and co-workers (J. Am. Chem. Soc. 2006, 128, 13702). The calculations indicate that the N-bound form is more stable. Further analysis reveals the presence of a CO ligand trans to the C/N binding site is a key factor in determining the greater stability of the N-bound form. This preference is further enhanced by the bulky iPr substituent at the N3 position. The calculations predict that the C-bound tautomer will be favoured with NHC ligands that feature a bulky C5 substituent in combination with small groups at N3 and C4. Thus [Ru(Cl)(H)(CO)(PPh3)2-(NHC)] complexes where NHC = 5-R-imidazol-2-ylidene or 3-Me-5-R-imidazol-2-ylidene (R = tBu, Ph) are predicted to be more stable as the C-bound form. Five-coordinate square- pyramidal species formed by loss of a CO or Cl ligand from 2 and 3 show an increased preference for the C-bound form. Indeed, when the C/N binding site is trans to a vacant site the C-bound tautomer becomes the more stable species. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009.
- Carbene ligands
- Density functional calculations
- N-Heterocyclic carbenes