C-H activation reactions by yttrium and lutetium hydride complexes:H/D exchange vs metalation of hydrocarbons. Importance of the hybridization state at the α carbon

Extended Hückel (EHT) calculations have been used to discuss the two alternative σ-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 Cp2Ln+ 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 fragment. H/D exchange is shown to be easier for R = aryl than for R = alkyl because an sp2 carbon can better stabilize the [H-R-H]- moiety than can an sp3 carbon. This is in agreement with the available experimental data where Ln = Sc, Y and Lu and is also related to the ability of an aryl moiety to stabilize nucleophiles as seen in Meisenheimer salts