N-Heterocyclic carbenes (NHCs) have emerged as one of the most useful class of ligands for metal complexes primarily due to their relatively easy synthesis, broad structural and stereoelectronic diversity, and their ability to form stable compounds with different metal ions. Among a vast number of variants known, annellated systems which feature fused ring(s) on the basic framework of NHC have drawn wide attention for their unique structural and stereoelectronic properties. The annellation has been attributed to have a significant effect on the ligating behavior of NHCs. The actual effect depends on the nature of the annellated system and the position in the parent scaffold where annellation takes place. The transition metal complexes bearing annellated NHCs have the potential to exhibit superior activity and/or selectivity in catalytic transformations. Besides, the fusion of ring(s) provides further scope for the functionalization of NHC. The NHCs annellated to suitably functionalized scaffolds have been used to design the stimuli-modulable systems. The use of NHCs fused to extended aromatic systems has paved the way for improving the efficacies of the catalytic systems using non-covalent interactions. Metal complexes containing annellated NHCs have afforded compounds with interesting photophysical and electronic properties. Herein we classify annellated NHCs based on their structural attributes. The effects of annellation on the ligand behavior of NHCs and the catalytic utility of metal complexes featuring these annellated carbenes, along with a host of other promising applications, are described.

N-杂环卡宾（NHC）已成为金属络合物中最有用的一类配体，这主要是由于它们相对易于合成，广泛的结构和立体电子多样性以及它们能够与不同的金属离子形成稳定化合物的能力。在众多已知的变体中，在NHC基本框架上具有稠环的无环系统因其独特的结构和立体电子性质而受到广泛关注。归因已被认为对NHC的结扎行为具有重大影响。实际效果取决于去核化系统的性质以及发生去核化作用的母支架中的位置。带有环化NHC的过渡金属络合物在催化转化中具有表现出优异活性和/或选择性的潜力。此外，环的融合为NHC的功能化提供了进一步的范围。与适当功能化的支架成环的NHC已经被用于设计可调节刺激的系统。将NHC与稠合的芳族体系融合使用为使用非共价相互作用提高催化体系的效率铺平了道路。包含环化NHC的金属配合物已提供具有令人感兴趣的光物理和电子特性的化合物。在这里，我们根据其结构属性对无环NHC进行分类。壬基化对NHCs配体行为的影响以及以这些壬基化卡宾为特征的金属配合物的催化作用，