DOI: 10.1002/cplu.201200025 [TeX 3 ] + Cations Stabilized by the Weakly Coordinating [Al(OR F ) 4 ] À Anion: FIR Spectra, Raman Spectra, and Evaluation of an Abnormal Halogen Dependence of the 125 Te NMR Chemical Shifts** Tobias A. Engesser, [a] Peter Hrobµrik,* [b, c] Nils Trapp, [a] Philipp Eiden, [a] Harald Scherer, [a] Martin Kaupp,* [b] and Ingo Krossing* [a] Dedicated to Professor Helge Willner on the Occasion of his 65th Birthday Introduction Over the last sixty years several syntheses of [TeX 3 ] + (X = F, Cl, Br, I) containing compounds were established. It was shown that it is necessary to stabilize the reactive cation with a weak, unreactive anion, for example [AlCl 4 ] À , [AsF 6 ] À , [SbF 6 ] À . The compounds were studied in the solid state through X-ray dif- fraction, IR, and Raman spectroscopy. The best studied cation is [TeCl 3 ] + for which the first Raman spectrum was published in 1954. [1] In 1967 the first IR spectra of TeCl 3 [A] ([A] À = [SbCl 6 ] À , [AsF 6 ] À , [PCl 6 ] À and [AlCl 4 ] À ) were measured by Beattie and Chudzynska [2] The first crystal structure of an [TeX 3 ] + salt ap- peared in 1971 with TeCl 3 [AlCl 4 ], [3] followed by TeF 3 [Sb 2 F 11 ]. [4] In 1980 and 1981 the crystal structures of the last two missing cations were published for TeBr 3 [AsF 6 ] and TeI 3 [AsF 6 ]. [5, 6] Collins and Schrobilgen were the first to use 125 Te NMR spectroscopy in 1985 and studied TeF 3 [AsF 6 ] in SO 2 . [7] Hitherto only [TeF 3 ] + and [TeBr 3 ] + were studied in solution with NMR spectrosco- py. [7, 8] Especially in the 1990s, many new crystal structures were published, sometimes with less common anions like [AuBr 4 ] À , [9] [Zr 2 Br 9 ] À , [10] [MoCl 4 O] À , [11] or [InI 4 ] À , [12] to name only a few. Most of the structures contain the [TeCl 3 ] + cation. For the [TeI 3 ] + cation several crystal structures, but no spectroscop- ic data were reported, probably because of the reported insol- ubility [13] or instability (solution 125 Te NMR) of the compounds known so far. Herein, we show that it is possible to stabilize [TeX 3 ] + (X = Cl, Br, I) with the very weakly coordinating anion [Al(OR F ) 4 ] À in the solid state and in SO 2 -solution to study their chemical shifts with 125 Te NMR and to measure a complete series of IR and Raman spectra of [TeX 3 ] + salts for the first time. The com- plete set of 125 Te NMR chemical shifts of the [TeX 3 ] + cations (X = Cl, Br, I) reveals a trend that conforms neither to the well- known “normal halogen dependence” (NHD) nor to the “in- verse halogen dependence” (IHD) found for many other nuclei. [14] The observations are analysed in terms of scalar rela- tivistic and spin-orbit effects by relativistic density functional calculations. TeX 3 [Al(OR F ) 4 ] (X = Cl, Br, I; R F = C(CF 3 ) 3 ) were synthesized by the reaction of Ag[Al(OR F ) 4 ] and TeX 4 or the reaction of AuX, Ag[Al- (OR F ) 4 ], and elemental tellurium in liquid SO 2 . The compounds were characterized by 125 Te NMR in solution and by X-ray dif- fraction, Raman, and IR spectroscopy in the solid state. The vi- brational spectra and the crystal structure show very weak sec- ondary interactions, indicating “pseudo gas phase conditions” in the condensed phase. The observed trend of the 125 Te NMR chemical shifts along the [TeX 3 ] + series follows neither the mo- notonous decrease known as “normal halogen dependence” nor the increase known as “inverse halogen dependence”. By relativistic two-component calculations based on the ZORA ap- proach, we find that this “abnormal halogen dependence” re- sults from an interplay of relativistic and solvent effects, where non-negligible scalar relativistic effects and intermediate-sized spin-orbit effects compensate to some extent. The reasons for these trends are evaluated in the context of the Te s-orbital character of the Te ÀX bonds and compared with the halogen dependence(s) within the isoelectronic [SeX 3 ] + and PX 3 series and related trihalomethyl [CX 3 ] + cations. [a] Dipl.-Chem. T.A. Engesser, Dr. N. Trapp, Dr. P. Eiden, Dr. H. Scherer, Prof. Dr. I. Krossing Institut für Anorganische und Analytische Chemie Freiburger Materialforschungszentrum (FMF) and Freiburg Institute for Advanced Studies (FRIAS), Section Soft Matter Science Universität Freiburg, Albertstrasse 21, 79104 Freiburg (Germany) Fax: (+ 49) 761-203-6001 E-mail : krossing@uni-freiburg.de [b] Dr. P. Hrobµrik, Prof. Dr. M. Kaupp Institut für Chemie, Theoretische Chemie Sekretariat C7 Technische Universität Berlin Strasse des 17. Juni 135,10623 Berlin (Germany) E-mail : martin.kaupp@tu-berlin.de [c] Dr. P. Hrobµrik Institute of Inorganic Chemistry, Slovak Academy of Sciences Dfflbravskµ cesta 9, 84536 Bratislava (Slovakia) E-mail : peter.hrobarik@savba.sk [**] X = Cl, Br, I and R F = C(CF 3 ) 3 Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201200025. ChemPlusChem 2012, 77, 643 – 651  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 643