Synthesis, redox chemistry and EPR spectroscopy of the mixed-sandwich complexes (eta-arene)(eta-cycloheptatrienyl)metal(z+) (M = Cr or Mo; z = 1 or 2): crystal structures of the redox pair [Cr(eta-C6H5Me)(eta-C7H6C6H4Me-4)][PF6](n) (n = 1 or 2)

The mixed sandwich complexes [M(eta -arene)(eta -C7H6R')](+) (M=Cr, R'=H, a rene=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)(e ta -C7H6R')](+) in the appropriate arene solvent. Reflux of [Mo(eta -C6H5Me )(eta -C7H7)](+) with an excess of HC=CBut in acetone affords [Mo(eta -C6H3 Bu3t-1,3,5)(eta -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 a t 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 An gstrom) consistent with a HOMO which is essentially non-bonding with respec t to the metal-ring interaction.