an aromatic species with the formula . Its name derives from the molecule tropine (itself named for the molecule atropine). Salts of the tropylium cation can be stable, e.g. tropylium tetrafluoroborate. It can be made from cycloheptatriene (tropylidene) and bromine or phosphorus pentachloride It is a planar, cyclic, heptagonal ion; it also has 6 π-electrons (4n + 2, where n = 1), which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms. The structure shown is a composite of seven resonance contributors in which each carbon carries part of the positive charge. In 1891 G. Merling obtained a water-soluble salt from a reaction of cycloheptatriene and bromine. The structure was elucidated by Eggers Doering and Knox in 1954. Another aromatic carbenium ion is the cyclopropenyl or cyclopropenium ion, , obtained by Ronald Breslow and John T. Groves in 1970. Though less stable than the tropylium cation, this carbenium ion can also form salts at room temperature. Solutions of such salts were found by Breslow and Groves to have spectroscopic and chemical properties matching expectations for an aromatic carbenium ion. Triphenylmethyl (trityl) cation The triphenylcarbenium or triphenylmethyl cation, , is especially stable because the positive charge can be distributed among 10 of the carbon atoms (the 3 carbon atoms in the ortho and para positions of each of the three phenyl groups, plus the central carbon atom). It exists in the compounds triphenylmethyl hexafluorophosphate , triphenylmethyl tetrafluoroborate , and triphenylmethyl perchlorate . Its derivatives include the triarylmethane dyes. Arenium ions An arenium ion is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate, or a σ-complex. Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring. The arenium ion is no longer an aromatic species; however it is relatively stable due to delocalization: the positive charge is delocalized over 5 carbon atoms via the π system, as depicted on the following resonance structures: Another contribution to the stability of arenium ions is the energy gain resulting from the strong bond between the benzene and the complexed electrophile. The smallest arenium ion is protonated benzene, . The benzenium ion can be isolated as a stable compound when benzene is protonated by the carborane superacid, H(CB11H(CH3)5Br6). The benzenium salt is crystalline with thermal stability up to 150