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, res pectively) which have the (TlBr5)(2-) anionic stoichiometry, but single cry stal 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 diprotonate d bipyridinium cations produce complexes (III and IV, respectively) which h ave 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 latte r distance is shorter than the sum of the van der Waals' radii for the Tl a nd 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.