Sterically demanding cycloheptatrienylium (tropylium) salts of the type (1,3,5-C₇H₄R₃)BF₄ [R = t-Bu, (3a)BF₄; R = SiMe₃, (3b)BF₄] have been prepared from the corresponding 1,3,5-trisubstituted benzene derivatives 1 by ring expansion with diazomethane followed by hydride abstraction with triphenylcarbenium tetrafluoroborate, (Ph₃C)BF₄. Complexation can be achieved by arene exchange and Mo(CO)₃ group transfer employing [(η⁶-p-xylene)Mo(CO)₃] (4) to yield the cationic complexes (5)BF₄. In refluxing mesitylene, [(η⁷-C₇H₄t-Bu₃)Mo(CO)₃]BF₄, (5a)BF₄, undergoes CO substitution to furnish the mesitylene sandwich complex (6a)BF₄. A cyclic voltammetric study reveals that this complex exhibits a reversible one-electron oxidation to the dicationic 17e complex 6a²⁺, which can also be accessed by chemical oxidation with AgBF₄. On the contrary, the reduction of 6a⁺ is irreversible and does not yield a stable 19e complex 6a. To study the fate of the reduced 19e form, (5a)BF₄ was treated with Na₂Hg to diastereoselectively afford the C–C coupled bicycloheptatriene complex 7a. Paramagnetic, dinuclear complexes of the type [(η⁷-C₇H₄R₃)Mo(μ-Cl)₃Mo(η⁷-C₇H₄R₃)] (8) have been obtained from the reaction of (5)BF₄ with Me₃SiCl. These can be regarded as mixed-valence Mo(0)/Mo(+I) compounds with a metal–metal bond order of 0.5. Cyclic voltammetric studies reveal that both complexes 8a and 8b undergo reversible one-electron oxidation as well as reduction. Treatment with one equivalent of ferrocenium hexafluorophosphate leads to removal of the unpaired electron and formation of the diamagnetic complexes (8)PF₆. Theoretical DFT calculations have been carried out to further elucidate the bonding in these systems. In addition, the X-ray crystal structures of (5b)BF₄, (6a)BF₄·CH₂Cl₂, (6a)(BF₄)₂·(acetone)₂, 7a·CH₂Cl₂, 8a·0.5C₆H₁₄, and (8a)PF₆·Et₂O are reported.