Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal–ligand framework. However, it becomes obvious that even metal species with a strong M–NHC bond can undergo evolution in catalytic systems, and processes of M–NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M–NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

中文翻译：

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R-NHC 偶联（R = C、H、杂原子）和 M-NHC 键断裂在 M/NHC 配合物演化和催化活性物质形成中的关键作用

金属与 N-杂环卡宾配体 (M/NHC) 的配合物通常被认为是均相催化中的首选系统，因为它们具有稳定的金属配体框架。然而，很明显，即使是具有强 M-NHC 键的金属物质也可以在催化体系中发生演化，并且 M-NHC 键断裂过程对于不同的金属和 NHC 配体来说是常见的。本综述重点关注 M-NHC 键断裂反应的主要类型及其对 M/NHC 催化体系活性和稳定性的影响。我们首次根据源自 M/NHC 预催化剂的 NHC 连接和 NHC 断开活性物质来考虑这些过程，并将它们分类为根本不同类型的催化剂。在两种不同类型的 M/NHC 催化机制的背景下讨论了合理的催化剂设计问题和可持续性问题。