Constrained carbon bonding inside fullerene cages

→ metal atoms). These bonding situations enrich the carbon bonding models beyond traditional organic chemistry. This review provides a comprehensive summary of the recent findings regarding constrained carbon bonding with varying numbers of carbon atoms inside carbon cages. It will encompass crucial aspects of this special constrained carbon bonding such as the dispersion of negative charge on the carbon cage, reduction of Coulomb repulsion, maximization of coordinated metal ions, and determination of optimal configurations for metal atoms within the carbon cages. Accordingly, new carbon bonding could be identified in carbon cages, which holds significant implications in the development of innovative carbon-based compounds. Additionally, the current challenges faced and future developments anticipated from the aspect of confined carbon bonding inside carbon cages will be discussed to provide deeper insights into the intricacies of carbon bonding. Through this comprehensive exploration, we hope to advance knowledge in this exciting area of carbon chemistry.