C-H ACTIVATION REACTIONS BY YTTRIUM AND LUTETIUM HYDRIDE COMPLEXES - H D EXCHANGE VS METALATION OF HYDROCARBONS - IMPORTANCE OF THE HYBRIDIZATION STATE AT THE ALPHA-CARBON/

Extended Huckel (EHT) calculations have been used to discuss the two a lternative sigma-bond C-H metathesis reactions which occur with organo -lanthanide (Ln = Y, Lu) compounds. The two reactions lead either to H /H (H/D) exchange or to metalation and have been modelled by studying the interaction of a Cp(2)Ln(+) fragment with [H-R-H](-) and [R-H-H](- ) respectively. It is shown that the metallic fragment interacts in a similar way with the two organic fragments and that the preference for one of the two paths is linked to the bonding within the organic frag ment. H/D exchange is shown to be easier for R = aryl than for R = alk yl because an sp(2) carbon can better stabilize the [H-R-H](-) moiety than can an sp(3) carbon. This is in agreement with the available expe rimental data where Ln = Sc, Y and Lu and is also related to the abili ty of an aryl moiety to stabilize nucleophiles as seen in Meisenheimer salts.