Synthesis of Chromium(0) and Molybdenum(0) Bis (?6-arene) Derivatives and Their Monoelectronic Oxidation to [M(?6-arene)2]+ Cations

M(g6-arene)2 species (M = Cr, arene = 1,3,5-Me3C6H3; M = Mo, arene = 1,3,5-Me3C6H3, 1,3,5-iPr3C6H3), have been prepared by a modified Fischer–Hafner synthesis or by metal vapour techniques. The reaction of Cr(g6-1,3,5-Me3C6H3)2 with the fulvene derivatives pentacarbomethoxycyclopentadiene (pcmcpH), 1-benzoyl-6-hydroxy-6-phenylfulvene (dbcpH), or 1-benzoyl-3-nitro-6-hydroxy-6-phenylfulvene (dbncpH) proceeds with evolution of dihydrogen and formation of the ionic derivatives [Cr(g6-1,3,5-Me3C6H3)2][E], where E = pcmcp, dbcp, or dbncp. Mo(g6-arene)2 derivatives (arene = toluene, 1,3,5-Me3C6H3, 1,3,5-iPr3C6H3) are oxidized to [Mo(g6-arene)2]+ by pcmcpH. The crystal and molecular structures of [M(g6-1,3,5-R3C6H3)2][pcmcp] (M = Cr, R = Me; M = Mo, R = Me, iPr) have been solved by X-ray single crystal diffraction.