Chemical and Electrochemical Reduction of Polyarene Manganese Tricarbonyl Cations:  Hapticity Changes and Generation of <i>Syn</i>- and <i>Anti</i>-Facial Bimetallic η<sup>4</sup>,η<sup>6</sup>-Naphthalene Complexes

(η<sup>6</sup>-Naphthalene)Mn(CO)<sub>3</sub><sup>+</sup> is reduced reversibly by two electrons in CH<sub>2</sub>Cl<sub>2</sub> to afford (η<sup>4</sup>-naphthalene)Mn(CO)<sub>3</sub><sup>-</sup>. The chemical and electrochemical reductions of this and analogous complexes containing polycyclic aromatic hydrocarbons (PAH) coordinated to Mn(CO)<sub>3</sub><sup>+</sup> indicate that the second electron addition is thermodynamically <i>easier</i> but kinetically slower than the first addition. Density functional theory calculations suggest that most of the bending or folding of the naphthalene ring that accompanies the η<sup>6</sup> → η<sup>4</sup> hapticity change occurs when the second electron is added. As an alternative to further reduction, the 19-electron radicals (η<sup>6</sup>-PAH)Mn(CO)<sub>3</sub> can undergo catalytic CO substitution when phosphite nucleophiles are present. Chemical reduction of (η<sup>6</sup>-naphthalene)Mn(CO)<sub>3</sub><sup>+</sup> and analogues with <i>one</i> equivalent of cobaltocene affords a <i>syn</i>-facial bimetallic complex (η<sup>4</sup>,η<sup>6</sup>-naphthalene)Mn<sub>2</sub>(CO)<sub>5</sub>, which contains a Mn−Mn bond. Catalytic oxidative activation under CO <i>reversibly</i> converts this complex to the zwitterionic <i>syn</i>-facial bimetallic (η<sup>4</sup>,η<sup>6</sup>-naphthalene)Mn<sub>2</sub>(CO)<sub>6</sub>, in which the Mn−Mn bond is cleaved and the naphthalene ring is bent by 45°. Controlled reduction experiments at variable temperatures indicate that the bimetallic (η<sup>4</sup>,η<sup>6</sup>-naphthalene)Mn<sub>2</sub>(CO)<sub>5</sub> originates from the reaction of (η<sup>4</sup>-naphthalene)Mn(CO)<sub>3</sub><sup>-</sup> acting as a nucleophile to displace the arene from (η<sup>6</sup>-naphthalene)Mn(CO)<sub>3</sub><sup>+</sup>. Heteronuclear <i>syn</i>-facial and <i>anti</i>-facial bimetallics are formed by the reduction of mixtures of (η<sup>6</sup>-naphthalene)Mn(CO)<sub>3</sub><sup>+</sup> and other complexes containing a fused polycyclic ring, e.g., (η<sup>5</sup>-indenyl)Fe(CO)<sub>3</sub><sup>+</sup> and (η<sup>6</sup>-naphthalene)FeCp<sup>+</sup>. The great ease with which naphthalene-type manganese tricarbonyl complexes undergo an η<sup>6</sup> → η<sup>4</sup> hapticity change is the basis for the formation of both the homo- and heteronuclear bimetallics, for the observed two-electron reduction, and for the far greater reactivity of (η<sup>6</sup>-PAH)Mn(CO)<sub>3</sub><sup>+</sup> complexes in comparison to monocyclic arene analogues.