This book focuses on the creation and demonstration of a carbon–carbon (C–C) single bond beyond 1.8 Å and on elucidation of its unique nature. C–C single bond is one of the most fundamental concepts in organic chemistry. The elucidation of its nature is important for further understanding chemical phenomena. The nature in the extreme state of C–C single bond is still unexplored because of the instability of compounds. In terms of its bond length, the limit had been predicted around 1.8 Å based on the experimental and theoretical studies. This book describes a first example of a C–C single bond beyond 1.8 Å by employing the original intramolecular core-shell strategy to make a weak and elongated bond stable enough. The presence of such an elongated bond was demonstrated by experimental and theoretical studies. The bond length changes could affect physical properties such as optical absorption and redox potential. Furthermore, its unique "flexibility" was discovered. This book benefits the chemists with deeper understanding of the covalent bonding.