Structural and Electronic Effects of Stepwise Reduction of a Tetraaryl[3]Cumulene

The chemical reduction of a 3]cumulene (3]TrTol) has been explored using alkali metals. Mono‐ and doubly reduced forms of 3]TrTol were isolated as solvent‐separated ion pairs with {Na(18‐crown‐6)THF₂}⁺ and {K(18‐crown‐6)THF₂}⁺ counterions and crystallographically characterized. This allowed analysis of structural parameters of the “naked” anions of 3]TrTol without interference from metal binding. The dianion of 3]TrTol was also isolated as a contact‐ion complex with {Cs(18‐crown‐6)}⁺ cations, thereby adding the effect of metal coordination to the core. Structural comparisons of anions to the neutral molecule, 3]TrTol, outline monotonic increases in bond‐length alternation (BLA) upon stepwise reduction. The greatest BLA value is found for the contact‐ion complex, which shows an alternating sequence of short and long carbon–carbon bonds, consistent with the structure of an alkyne. In contrast to studies on tetraphenyl3]cumulene, the cumulenic framework of 3]TrTol remains planar in all the derivatives.