Tl-X bonds

The broken bonds (boldface = dissociated fragm BDEs (boldface = recommended data reference in parentheses) Methods (reference in References 

---

When oxygen, nitrogen, and sulfur bound to organic groups are also bound to Tl1, the Tl—X bond appears to be more covalent than the bond to alkali metal ions in similar compounds. Thallium compounds tend to be polymeric rather than ionic. Thus the acetylacetonate is a linear polymer with 4-coordinate Tl. 

The gas-phase molecular structures and dissociation energies of the seven monohalides MCI in Table 13 have all been determined by high-temperature spectroscopic studies. As expected, M-Cl bond distances increase, and the dissociation energies decrease as Group 13 is descended. Similarly Tl-X bond distances in the three thallium monohalides increase and the bond energies decrease as the halogen group is descended. 

Metal cations can lend electrophilic assistance to weaken the Pd—X bonds in the intermediate R-Pd —X. Either full fission of this bond, leading to the realization of a polar mechanism, or partial polarization, might take place. Soft Lewis acids (the cations of Cu, Ag, Tl) are used most often (see Chapter 9.8 for a discussion of how metal ions act as Lewis-acid catalysts). 

Because of the 6s inert-pair effect, many T1+ compounds are found to be more stable than the corresponding Tl3+ compounds. The progressive weakening of M3+-X bonds from B to Tl partly accounts for this. Furthermore, the relativistic effect (as described in Section 2.4.3) is another factor which contributes to the inert-pair effect. 

The complex TlClg is formed both in aqueous solution and in the solid state. Its structure is octahedral, as shown from the IR/Raman and X-ray diffraction data (115,136) the Tl-Cl bond distance is 2.59(1)... 

Parenthetically, it should also be noted that allylic and benzylic substrate are particularly reactive in SN2 reactions as well as in SN1 reactions. Allvli. and benzylic C-X bonds are about 50 kJ/mol (12 keal/mol) weaker than tl corresponding saturated bonds and are therefore more easily broken. 

Supramolecular self-organization in the solid state has been observed (by X-ray diffraction) for several thallium-oxygen compounds. The secondary Tl - O bonds in these compounds are rather strong and it is debatable whether these should be regarded as electron-pair donor-acceptor bonds or as secondary bonds. 

---