The cross-metathesis reaction pathways of norbornene (NB) with ethylene (2a), trans-1,2-difluoro-ethylene (2b) and trans-1,2-dichloro-ethylene (2c) using (1,3-diphenyl-4,5-dihydroimidazol-2-ylidene) (PCy3)CI2Ru=CHPh (1) have been studied at B3LYP/LACVP(*) level of theory. The calculations show that the Gibbs free activation energy of the metathesis reaction is strongly dependent on the volume of halogene substituents. The calculated Gibbs free activation energies for cross-metathesis of NB with 2a, 2b and 2c are of 10.2, 17.3 and 25.1 kcal/mol, respectively. Strong binding in transition states results in large deformation energies increasing energy barrier. Calculation shows that steric hindrance caused by the atoms directly linked to the olefin double bond is the most important factor influencing the activation energy of the metathesis reaction. (c) 2006 Elsevier B.V. All rights reserved.