The precise identification of metal-metal bonds is critical to fully understanding the nature of metalmetal bonding but remains a fundamental challenge. Herein, we show the essence of Sc-Sc bonds with a metal-metal distance of 3.36 Å in a C 3v (8)-C 82 fullerene cage using crystallography. To elucidate the metal-metal bond formation, a metallofullerene Sc 2 C 2 @C 3v (8)-C 82 with a Sc 2 C 2 endo-unit encapsulated inside the same fullerene[82] cage was studied as a unique control molecule. Theoretical calculations reveal that each Sc atom donates one electron to the Sc-Sc bond. The existence of the Sc-Sc bond in Sc 2 @C 3v (8)-C 82 versus Sc 2 C 2 @C 3v (8)-C 82 is thoroughly investigated.

Endohedral metallofullerenes (EMFs) are sub-nano carbon materials with diverse applications, yet their formation mechanism, particularly for metastable isomers, remains ambiguous. The current theoretical methods focus mainly on the most stable isomers, leading to limited predictability of metastable ones due to their low stabilities and yields. Herein, we report the successful isolation and characterization of two metastable EMFs, Sc2C2@C 1(39656)-C82 and Sc2C2@C 1(51383)-C84, which violate the isolated pentagon rule (IPR). These two non-IPR EMFs exhibit a rare case of planar and pennant-like Sc2C2 clusters, which can be considered hybrids of the common butterfly-shaped and linear configurations. More importantly, the theoretical results reveal that despite being metastable, these two non-IPR EMFs survived as the products from their most stable precursors, Sc2C2@C 2v (5)-C80 and Sc2C2@C s (6)-C82, via a C2 insertion during the post-formation annealing stages. We propose a systematic theoretical method for predicting metastable EMFs during the post-formation stages. The unambiguous molecular-level structural evidence, combined with the theoretical calculation results, provides valuable insights into the formation mechanisms of EMFs, shedding light on the potential of post-formation mechanisms as a promising approach for EMF synthesis.

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