published as an Advance Article on the web 24th January 2002 DFT(B3PW91) calculations of the activation of CH 4 by models (Cl 2 LnZ) of Cp* 2 LnZ (Z = H, Me) have been carried out for the entire lanthanide series. Cl 2 LnZ appears to be a good model for Cp* 2 LnZ. It reproduces well the coordination around the lanthanide. The energetics of the transformation X 2 LnH ϩ CH 4 X 2 LnCH 3 ϩ H 2 are fairly close for X = Cl and Cp and the difference in behavior can be attributed to the stronger electron donating ability of Cp. Formation of the lanthanide hydride complex is calculated to be exothermic in agreement with experimental evidence. The energy profiles of the reactions Cl 2 LnH ϩ CH 4 Cl 2 LnCH 3 ϩ H 2 ; Cl 2 LnH* ϩ CH 4 Cl 2 LnH ϩ H*CH 3 ; Cl 2 LnCH* 3 ϩ CH 4Cl 2 LnCH 3 ϩ H-CH* 3 have been calculated. The transition states for the first and third transformations are energetically accessible, in good agreement with the known experimental data. The second reaction has a transition state of very high energy indicating an unfeasible reaction. The geometry of the transition stuctures are suggestive of a proton transfer between two anionic species (Z and CH 3 Ϫ ; Z = H Ϫ and CH 3 Ϫ ) in the field of the lanthanide fragment.
This paper discusses some relationships between the electronic structure and the reactivity of lanthanide complexes. The electronic structures of some representative lanthanide complexes are described. The 4f electrons do not participate in the Ln-X bond and a comparison between lanthanide and d 0 transition metal complexes from Groups 3 and 4 highlights the dominant ionic character in the bonding to lanthanide centers. However some covalent character cannot be excluded and this covalent character rationalizes the geometry of the complexes such as the non-planar structure of LaX 3 (X = H, Me, F). The consequences of the nature of the bonding on the β Si-C agostic interaction in La{CH(SiMe 3 ) 2 } 3 is presented. The unusual O-bonding mode of CO to Cp* 2 Yb is briefly summarized. The reactivity of X 2 Ln-Z (X = Cp, H, Cl, effective group potential; Ζ = H, Me) with simple molecules like H 2 , CH 4 , and SiH 4 is compared. It is shown that the strong ionic character of the lanthanide bonding is key to rationalizing the selectivity of the σ-bond metathesis with alkane and to the lack of it in the case of silane. In particular, the position β to the metal center in the diamond shape 4c-4e -transition state is not allowed for a methyl group (transition state of very high energy) whereas it is permitted to a silyl group. This is shown to be related to the relative ability of carbon and silicon to be hypervalent.
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