The mixed sandwich complexes [M(η-arene)(η-C₇H₆R′)]⁺ (M = Cr, R′ = H, arene = C₆H₄Me₂-1,4, 1a; or C₆H₃Me₃-1,3,5, 2a; R′ = C₆H₄Me-4, arene = C₆H₅Me, 3a; M = Mo, R′ = H, arene = C₆H₃Me₃-1,3,5, 4a) were prepared by reflux of [M(CO)₃(η-C₇H₆R′)]⁺ in the appropriate arene solvent. Reflux of [Mo(η-C₆H₅Me)(η-C₇H₇)]⁺ with an excess of HCCBu^t in acetone affords [Mo(η-C₆H₃Bu^t₃-1,3,5)(η-C₇H₇)]⁺, 5a. Cyclic voltammetric studies in NCMe reveal that each of 1a, 2a, 3a and 5a undergoes reversible one-electron oxidation processes to give the corresponding, isolable 17-electron radical dications, 1b, 2b, 3b and 5b which have been characterised by EPR spectroscopy. NMR data for the 18-electron monocations suggest an enhanced electron density at the arene ring in the chromium derivatives by comparison with molybdenum analogues and this is reflected in the stability of complexes 1a, 2a and 3a towards arene displacement reactions. The crystal structure of 3a reveals only a small asymmetry in the average chromium-to-ring carbon bond lengths for the arene and cycloheptatrienyl rings. One-electron oxidation of 3a to give 3b results in a small increase in metal-to-ring distances (ca. 0.02 Å) consistent with a HOMO which is essentially non-bonding with respect to the metal–ring interaction.