A system of additive covalent radii is proposed for sigma(2) pi(4) triple bonds involving elements from Be to E 112 (eka-mercury). Borderline cases with weak multiple bonding are included. Only the elements in Group 1, the elements Zn-Hg in Group 12 and Ne in Group 18 are then totally excluded. Gaps are left at late actinides and some lanthanides. The standard deviation for the 324 included data points is 3.2 pm.

In this review we discuss the recent discoveries in polyhalide anion chemistry with the main focus on polybromide, ‐chloride, and ‐fluoride anions. Based on novel synthetic strategies either in ionic liquids or in neat halogens several new polyhalides of bromine and chlorine were synthesized. Beyond these discoveries the chemistry of polyfluoride monoanions is reviewed. Such species were investigated under cryogenic conditions at 4 K in conjunction with quantum‐chemical calculations. In addition to these bonding and structural discussions, an overview of possible applications of such polyhalide anions is provided.

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One of the most important classifications in chemistry and within the periodic table is the concept of formal oxidation states. The preparation and characterization of compounds containing elements with unusual oxidation states is of great interest to chemists. The highest experimentally known formal oxidation state of any chemical element is at present VIII, although higher oxidation states have been postulated. Compounds with oxidation state VIII include several xenon compounds (for example XeO4 and XeO3F2) and the well-characterized species RuO4 and OsO4 (refs 2-4). Iridium, which has nine valence electrons, is predicted to have the greatest chance of being oxidized beyond the VIII oxidation state. In recent matrix-isolation experiments, the IrO4 molecule was characterized as an isolated molecule in rare-gas matrices. The valence electron configuration of iridium in IrO4 is 5d(1), with a formal oxidation state of VIII. Removal of the remaining d electron from IrO4 would lead to the iridium tetroxide cation ([IrO4](+)), which was recently predicted to be stable and in which iridium is in a formal oxidation state of IX. There has been some speculation about the formation of [IrO4](+) species, but these experimental observations have not been structurally confirmed. Here we report the formation of [IrO4](+) and its identification by infrared photodissociation spectroscopy. Quantum-chemical calculations were carried out at the highest level of theory that is available today, and predict that the iridium tetroxide cation, with a Td-symmetrical structure and a d(0) electron configuration, is the most stable of all possible [IrO4](+) isomers.

Laser-ablation experiments with metals provide a source of electrons for capture processes, which are codeposited with solid argon and neon containing molecular fluorine. New argon and neon matrix absorptions at 510.6 and 524.7 cm(-1), respectively, are photosensitive upon irradiation at >290 nm, which is consistent with their assignment to an isolated anion. These bands are below the [M](+)[F(3)](-) antisymmetric trifluoride stretching frequency of 550 cm(-1) in an argon matrix, which is the typical relationship for cation-anion complexes and matrix-isolated anions. Thus, we report the isolated [F(3)](-) anion in solid argon and neon environments. Moreover, we have carried out quantum-chemical calculations up to and including the CCSD(T) method to investigate the stabilities of polyfluoride anions higher than the [F(3)](-) anion.

This Review deals with the evolving field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides). In addition to a historical outline, current progress in synthetic approaches towards the formation of polyfluorides, polychlorides, polybromides, and polyinterhalides is also illustrated. The structural diversity of polyhalides has substantially increased in the past decade, especially for polychlorides and polybromides, which are commonly characterized by single‐crystal X‐ray diffraction, Raman spectroscopy, and quantum‐chemical calculations. Polyfluorides have been examined by sophisticated state‐of‐the‐art quantum‐chemical calculations and investigated spectroscopically in noble gas matrix‐isolation experiments under cryogenic conditions at 4 K. The bonding in such polyhalide systems is also discussed. The last Section deals with applications of polyhalides in halogenation reactions and electrochemistry as well as their use as reactive ionic liquids, emphasizing the promising future of polyhalogen chemistry.

The new Lewis acid Al(OTeF ) and its acetonitrile adduct CH CN→Al(OTeF ) were obtained by a simple one-step synthesis in batches of up to 15 g. Al(OTeF ) and the adduct were characterized by vibrational spectroscopy (IR, Raman) and quantum-chemical calculations. Furthermore, five different salts of the new weakly coordinating anion [Al(OTeF ) ] were prepared in a two-step procedure. [Ph P][Al(OTeF ) ], Cs[Al(OTeF ) ], [Ph C][Al(OTeF ) ], as well as the protonated benzene derivatives [C H ][Al(OTeF ) ] and [C H ][Al(OTeF ) ] were characterized by low-temperature single-crystal X-ray diffraction and NMR spectroscopy. Arenium salts have rarely been characterized in the solid state and were synthesized in this work in a simplified fashion.

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