Abstract
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)(PPh~3~)~2~(I__i__PrMe~2~)] (I__i__PrMe~2~ = 3‐isopropyl‐4,5‐dimethylimidazol‐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 __i__Pr 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)(PPh~3~)~2~(NHC)] complexes where NHC = 5‐R‐imidazol‐2‐ylidene or 3‐Me‐5‐R‐imidazol‐2‐ylidene (R = __t__Bu, 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)