[(η 6 -Trindane)Mn(CO) 3 ][BF 4 ] (1) reacts with tert-BuOK in THF in the presence of alkyl halides (CH 2 =CHCH 2 Br, CH 3 I) to yield the corresponding (η 6 -C 15 H 18 )Mn(CO) 2 X, where X = Br (3) or I (4). In contrast, 1 and tert-BuOK react, in the presence of a donor ligand, to generate (η 5 -C 15 H 15 )Mn(CO) 2 L, where L = CO (7), P(OMe) 3 (8), or PPh 3 (9), in which the metal has migrated from the central ring onto a peripheral ring that has lost three hydrogens; [(η 6 -indane)Mn(CO) 3 ][BF 4 ] behaves similarly. A mechanism is proposed in which the initially formed (η 6 -trindane)Mn(CO) 2 CO 2 -tert-Bu suffers the loss of CO 2 and isobutene to yield the hydride, (η 6 -trindane)Mn(CO) 2 H, that in turn undergoes three successive C-H insertions with concomitant loss of two molecules of dihydrogen. In contrast, [(η 6 -tetralin)Mn(CO) 3 ][PF 6 ] (17) and [(η 6 -dibenzosuberane)Mn(CO) 3 ][BF 4 ] (23) each suffer deprotonation at a benzylic site. In the first case, the zwitterion formed attacks its progenitor to yield a tetralenyl-substituted (η 5 -cyclohexadienyl)Mn(CO) 3 complex (21); however, for the dibenzosuberane complex (23) the elimination product (28) is stable. X-ray crystal structures are reported for 3, 4, 7, 8, 23, 28, and for the phosphite complexes [(η 6 -trindane)Mn(CO){P(OMe) 3 } 2 ][BF 4 ] (15) and [(η 6 -tetralin)Mn(CO) 2 {P(OMe) 3 }][PF 6 ] (20).Key words: arene-manganese, organometallic cations, deprotonations, X-ray crystallography.