Cross-metathesis reaction pathways of ethylene (2a), trans-1,2-dichloro-ethylene (2b) and fluorinated olefins trans-1,2-difluoro-ethylene (2c) and tetrafluoro-ethylene (2d), with norbornene (NB) using a (1,3-diphenyl-4,5-dihydroimidazol-2-ylidene) (PCy3)Cl2Ru = CHPh (1) have been studied at B3LYP/LACVP* level of theory. The calculated Delta G of reactions for olefins 2a, 2b, 2c and 2d were of 2.5, -2.0, -11.9 and -31.6 kcal/mol, respectively. The calculations show that the natural charge at a Ru center is strongly dependent on substituent nature and can be a measure of the carbene stability. The stabilization of a metallacarbene is due to the stabilization of the metal center and not a carbene carbon itself. (C) 2008 Elsevier B.V. All rights reserved.