Structural diversity in thallium chemistry - II. Potential pentabromothallate(III) derivatives: examples of incipient bromide ion coordination?

From aqueous, acidic thallium(III) bromide solutions, 1,10-phenanthrolinium and 4,4'-dimethyl-2,2'-bipyridinium cations yield complexes (I and II, respectively) which have the (TlBr5)(2-) anionic stoichiometry, but single crystal X-ray structure analyses show that both contain [TlBr4](-) (confirmed by their Raman spectral and hydrogen-bonded Br-. On the other hand, similar bromothallate(III) preparations employing the piperazinium and diprotonated bipyridinium cations produce complexes (III and IV, respectively) which have unusual 5-coordination at the metal, with four 'normal' Tl-Br distances of ca. 2.6 Angstrom and one longer contact of ca. 3.77 Angstrom. The latter distance is shorter than the sum of the van der Waals' radii for the Tl and Br atoms. Recrystallization of IV from ethanol causes HBr loss, yielding 2,2'-bipyridinium tetrabromothallate [C10H9N2](+) [TlBr4](-) (V). (C) 1999 Elsevier Science S.A. All rights reserved.