NHC-metal bond strengths

Transition-metal oxides are useful oxidizing reagents for organic molecules and often participate in oxygen-atom transfer reactions [21]. A prototypical example is CH3Re03 (MTO), which serves as a versatile reagent for stoi- 

Since this initial report, NHCs have been used to stabilize a number of additional high-valent metal complexes bearing oxo and nitrido ligands (Chart 1) [26-32]. In contrast to the MTO example, these complexes and the high-valent metal precursors employed in their synthesis are not especially strong oxidants. Consequently, the preparation of these complexes often can be achieved by simple addition of the NHC to an unsaturated metal center or via displacement of a weakly coordinated solvent molecule such 

Several interesting features have been noted in the studies of complexes 3-10. Cationic Mo(VI) complexes of the type [Mo02C1L3]C1 were not known until the synthesis of 6 [26]. With other ligands, including DMF, 

Chart 1 NHC-coordinated high-valent metal complexes 

In addition to the metal-oxo complexes, several examples of NHC-stabilized rhenium(V)-nitrido complexes exist, e.g., 9 and 10 (Chart 1) [28]. These adducts, which feature triazole-based ligands, were prepared via displacement of phosphine ligands. The stability of both phosphine and triazole-based carbenes in these reactions suggest the nitrido ligand is relatively unreactive. 

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