In this paper, we extend the work of Popelier and Logothetis [J. Organomet. Chem. 1998, 555, 101] on the characterization of agosticity by considerably enlarging the set of the studied organometallic molecules. To this aim, 23 representative complexes have been considered, including all first line transition metals at various oxidation states and exhibiting four types of agosticity (α, β, γ, and δ). From these examples, the concepts of agostic atom, agostic bond, and agostic interaction are defined and discussed, notably by advocating Bader's analysis of the electron density. The nature and the local properties of the bond critical points are then investigated, and the relationships with the main geometric parameters of the complexes are particularly examined. Moreover, new local descriptors based on kinetic energy densities are developed in order to provide new tools for bond characterization.

We report a theoretical study, based on density functional theory, devoted to the comprehension of the oligomerization reaction of 1-butene catalyzed by the Fe(II) bis(arylimino)pyridine complex ({2,6-(2-(CH3)C6H4NC(CH3))2-C5H3N}Fe(II)Cl2). Starting from the activated species ([{2,6-(2-(CH3)C6H4NC(CH3))2-C5H3N}Fe(III)H]2+), the obtained results are compared with available experimental data in order to determine the reaction mechanisms and to explain the unexpected predominant formation of linear octenes. All reaction steps (insertions, terminations) take place on the quartet potential energy surface, which appears to be the most stable all along the reaction path. The unique reaction path toward linear octenes (i.e., (1,2) and (2,1) successive insertions of 1-butene) is found as the most kinetically favorable to the detriment of all other possible paths. The first insertion into the Fe-hydride catalyst is found to be strongly exergonic and thermodynamically directed. Dimers can be formed by either β-hydrogen-elimination termination or β-hydrogen-transfer termination. However, termination reactions become competitive and favorable only for the third monomer insertion reactions, which is consistent with experimental data. The β-hydrogen-transfer termination reactions are largely dominant with respect to terminations by β-hydrogen elimination. A kinetic model is proposed to compare quantitatively the ratios of different generated species, and the obtained results are in good agreement with the experimental data.

International audienceAn exhaustive study of the neutral structural defects of icosahedral B4C has been performed with the density functional theory. Vacancies have been determined to be boron vacancies in the C-B-C chains. Their presence is shown to yield a discontinuous variation of crystal volume upon increasing pressure, when the formation of a C-C bond occurs in the chains. The dynamical failure of shocked B4C is attributed to the formation of these C-C bonds