Group 6 Metal NHC and CAAC Complexes in Homogeneous Catalysis: Reactivity Trends and Selected Applications

ABSTRACT Complexes of chromium, molybdenum, and tungsten bearing N ‐heterocyclic carbene (NHC) ligands have emerged as versatile platforms for homogeneous catalysis, combining the strong σ‐donor properties and structural tunability of NHCs with the availability and rich redox chemistry of these metals. Over the past decade, NHC ligation has enabled access to well‐defined Group 6 complexes spanning a wide range of oxidation states and coordination environments, including low‐valent carbonyl species, chelating bis‐NHC frameworks, and high‐valent oxo and nitride derivatives. These systems have been explored in a variety of catalytic transformations, for example, reduction, dehydrogenation, and oxidation. This review provides an overview of the development of Group 6 metal‐NHC complexes in homogeneous catalysis, focusing on the relationship between catalyst structure, ligand design, and reactivity. Emphasis is placed on mechanistic insights that illustrate how NHC ligands influence catalyst stability, activation pathways, and selectivity across different reaction classes. Rather than providing exhaustive coverage, the discussion highlights representative examples that define current capabilities and limitations, and identifies design principles and challenges that are likely to shape future advances.