Computational Study of the C ‐ and N ‐Bound Tautomers of [Ru(Cl)(H)(CO)‐(PPh 3 ) 2 (I i PrMe 2 )] (I i PrMe 2 = 3‐Isopropyl‐4,5‐dimethylimidazol‐2‐ylidene)

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)