Constrained carbon bonding inside fullerene cages

Carbon is an incredibly versatile element and can form bonds via sp, sp², and sp³ hybridization, forming diverse structures, which are responsible for the vast complexity and diversity of chemistry and biology. Therefore, understanding carbon bonding is crucial for comprehending the fundamental principles of natural science. Beyond conventional chemistry, carbon bonding confined inside carbon cages can adopt unusual and seemingly unpredictable bond states. Within these spatially restricted environments, encapsulated carbon atoms can bond with multiple nonmetal atoms (e.g., H, C, N, and O) and a variety of metal atoms (e.g., Sc, V, Ti, and Dy), forming otherwise unstable clusters with different bonding models and oxidation states of carbon. This leads to unprecedented bonding situations, including multiple and multicenter carbon-metal bonds, covalent carbon-metal bonds, superatomic states, and pronounced donation bonds (e.g. C₂ → 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.