Structural and Physical Properties of Delocalized Mixed-Valent [CpM (pentalene)M'Cp]ⁿ⁺and [CpM (indacene)M'Cp]ⁿ⁺ (M, M' = Fe, Co, Ni; n = 0, 1, 2) Complexes

The structural characterization and physical properties of [CpM (pentalene)M'Cp]ⁿ⁺ (Cp = penta- methylcyclopentadiene; M, M' = Fe, Fe (1a); Co, Co (1b); Ni, Ni (1c); Ru, Ru (1d); Fe, Ru (1e); Fe, Co (1f); n = 0, 1, 2) and [CpM (s-indacene)M'Cp]ⁿ⁺ (s = symmetric) and [CpM (as-indacene)M'Cp]ⁿ⁺ (M, M' = Fe, Fe (2a, 3a); Co, Co (2b, 3b); Ni, Ni (2c, 3c); n = 0, 1, 2) (as = asymmetric) are reported. The local molecular structure of the organometallic complex does not change significantly with oxidation state; in all cases the CpM moieties reside on opposite faces of the fused μ-bridging ring systems, reflecting the dominance of steric effects. These complexes generally exhibit behavior consistent with significant electronic interactions between metal centers, including large electrochemical potential separations between successive one-electron redox events, and for the mixed valent (n = 1+) complexes, intervalent charge transfer absorption bands. The magnetic susceptibility data are consistent with intramolecular ferromagnetic coupling of spins for 1a²⁺ and 2c²⁺ and antiferromagnetic coupling of spins for 1c, 1c²⁺, 1b, 2b, 1c⁺, 2c⁺, and 3c⁺. In general, the paramagnetic complexes exhibit Curie-Weiss behavior, except for 2c and 3c, which possess singlet ground states and high spin excited states that are 0.036 and 0.056 eV (290 and 524 cm^-1) above the ground state, respectively. Mixed-valent 1a⁺ and 2a⁺ are fully detrapped on the Mossbauer time scale (i.e., electron transfer rates ≥10⁷ s^-1) above 1.5 K, consistent with a negligible energy barrier to intramolecular electron transfer or complete delocalization. The EPR spectra of 1a⁺, 2a⁺, and 1e⁺ exhibit significantly reduced g-factor ansiotropies and more intense spectral features at ambient temperature compared to [FeCpa]^·+, implying intramolecular electron transfer rates ≥10⁷ s^-1.