Exceptionally Long C−C Single Bonds in Diamino‐o‐carborane as Induced by Negative Hyperconjugation

The synthesis of a series of 1,2‐diamino‐o‐carboranes (1–4) is reported. The molecular structures of these diamino‐o‐carboranes are remarkable as the inner‐cluster C−C bonds are all ultra‐long (162.7–193.1 pm) and vary substantially with small variations in the substituents. The results of quantum mechanical investigations suggest that the origin of the bond elongation is significant in‐plane negative hyperconjugation of lone pairs of the nitrogen substituents with the σ* orbitals of the C−C bonds in o‐carboranes. Breaking the law: A series of 1,2‐diamino‐o‐carboranes with exceptionally long C−C bonds (up to 193 pm) were synthesized. The origin of this bond elongation was attributed to the in‐plane negative hyperconjugation between the lone pairs of the nitrogen atoms and the σ*C−C orbital in the o‐carborane.