Oxidation of CS2 by AsBr4+: The unexpected formation of the simple CS2Br3+ carbenium ion

During the prepn. of AsBr4+[Al(OR)4]-, the novel carbocation CS2Br3+ was synthesized by reaction of AsBr3, Br2, CS2, and Ag[Al(OR)4] (R=C(CF3)3). CS2Br3+ [Al(OR)4]- was characterized by its crystal structure, NMR and IR spectroscopy, and quantum chem. calcns. (including COSMO solvation enthalpies). Addnl. expts. as well as the computed thermodn. indicated two likely reaction pathways: Ag+ + 2Br2 + CS2->CS2Br3+ + AgBr and the direct 4e- oxidn. reaction AsBr4+ + CS2->CS2Br3+ + 1/6As6Br6. Both reactions were obsd. exptl. and are exergonic in soln. by -226 and -56 kJ mol-1 resp. As a result of charge delocalization the C-S and C-Br distances in the cation are shortened by 0.06 to 0.08 .ANG.; the S-Br distances are also slightly shortened indicating a delocalization of the charge also to the bromine atoms in the (S-)Br moieties. Based on an anal. of the cation-anion contacts as well as quantum chem. MP2 calcns., a delocalization model as a planar 10 p electron system is discussed and the p MOs are given. The electronic situation of CS2Br3+ is very close to that in CBr3+, i.e., the properties of SBr moieties and Br atoms as p donors towards a formal C+ center are comparable. [on SciFinder (R)]