The reaction of ethylene with imines derived from alpha -naphthylcarbaldehyde catalyzed by Ru-3(CO)(12) leads to the selective and quantitative formation of products in which one molecule of ethylene has been inserted into the C-H bond in ortho position with respect to the exocyclic imine substituent. The stoichiometric reaction of the same ligands with Ru-3(CO)(12) leads to dinuclear ruthenium carbonyl complexes showing the same regioselectivity of C-H activation but the hydrogen atom is shifted in an intramolecular hydrogen transfer reaction towards the former imine carbon atom. If the catalytic alkylation of alpha -naphthylcarbaldimines is monitored by NMR the occurrence of the dinuclear product of the stoichiometric reaction is observed before the reaction again quantitatively yields the imines bearing an ethyl group in 2-position of the naphthalene core. This proofs that there must be an equilibrium between the dinuclear ruthenium carbonyl complex which is also observed if alpha -naphthylcarbaldimines are treated with an equimolar amount of Ru-3(CO)(12) and another ruthenium compound where the ethylene might be inserted catalytically into a ruthenium carbon bond.