Theoretical studies on the stability of thallium-carbon .sigma.-bonds in aliphatic organothallium compounds

Calculations of the electronic structure and bonding in organothallium compounds (T1R, T1R⁺, T1R<inf>2</inf>, T1R<inf>2</inf>⁺, T1R<inf>3</inf>, T1R<inf>4</inf>⁻, T1R<inf>2</inf>Br, and T1R' with R = CH<inf>3</inf>, R' = C<inf>2</inf>H<inf>3</inf>, or C<inf>2</inf>H), in the hydrides (R = H), their dissociation products (CH<inf>3</inf>, C<inf>2</inf>H<inf>3</inf>, C<inf>2</inf>H, C<inf>2</inf>H<inf>6</inf>, and T1Br), and T1<inf>2</inf> have been carried out. A modified version of Pople's SCF program GAUSSIAN82 has been used, in which the core electrons on the T1 atom are described by a quasi-relativistic [Pt]-core pseudopotential. All bond distances and angles have been optimized with a Fletcher-Powell procedure. SCF—SW—Xα calculations have been carried out to determine the extent of the Tl 5d core orbital participation in the Tl-C bond. Vibrational frequencies have been calculated from the harmonic diagonal SCF valence force field. As a result, the previously undetected molecule T1CH<inf>3</inf> is found to be stable with respect to dissociation into Tl and CH<inf>3</inf>. The isolated T1(CH<inf>3</inf>)<inf>3</inf> molecule is found to possess the expected planar-trigonal (T1C<inf>3</inf>) structure. It is explained why organothallium chemistry is mainly the chemistry of T1(III) in contrast to inorganic thallium chemistry and that T1—T1 bonds are very weak or do not exist is a relativistic effect.