Y. Jean

Ligand dependent nature of three possible shapes for a d6 pentacoordinated complex

Abstract The distortion of model XMH 4 d 6 complexes (X-equatorial ligand) away from their unstable trigonal bipyramidal (TBP) structure is studied by means of Extended Huckel calculations. The distortion is represented by the angle α between the two equatorial MH bonds (α = 120° in the TBP geometry). Departure from α = 120° leads to two minima, one for α = 180° (square pyramid geometry) and one for α = 70°, the origin of both being traced to a first order Jahn—Teller effect and not to a favourable ligand—ligand interaction (α = 70°). The relative energies of these two minima are strongly dependent on the nature of the equatorial ligand X opposite to the α angle. In particular it is shown that a π donor X ligand favours α = 70° while a π acceptor favours α = 180°. Finally a third minima is found for α = 30°. In this case, there is a strong interaction between the equatorial hydrides and this structure is better viewed as a d 8 square planar complex with a molecular H 2 ligand side-bonded to the metal.

Theoretical study of electrocyclic ring opening of oxirane.

Abstract SCF+CI calculations are performed to investigate the potential energy surface associated with the thermal C-C ring opening of oxirane (CH 2 OCH 2 ). Conrotatory, disrotatory and non-synchronous processes are studied, and the results discussed with respect to experimental data (Huisgen and toll.) which indicate a loss of stereoselectivity in the ring opening reactions of some substituted oxirane molecules.

Theoretical study of the conformations of cis carbene-acetylene transition metal complexes

Abstract We have analyzed the conformations of cis carbene-acetylene complexes by means of extended Huckel calculations. We have shown that the reactive conformation is not far in energy from the optimum conformation and is thus easily reached. This originates from the importance of the M-L/ligand interaction, associated with the presence of 4 CO ligands on the metal, in determining the conformation of the complex. A π donating group on the carbene makes the reactive conformation even more accessible. Replacing the CO ligands by ligands with no π accepting capability should make the carbene-acetylene complex more stable and less reactive.