Fischer and Schrock carbenes have M=C bonds that display highly deshielded carbon chemical shifts (greater than 250 ppm), in particular Fischer carbenes (greater than 300 ppm). They are typically described according to the electrophilic and nucleophilic nature of their metal-carbon double bonds. Recently, we have shown that the analysis of ¹³C NMR chemical shift anisotropy (CSA), defined by its principal components δ_ii (δ_ii = δ₁₁, δ₂₂, δ₃₃), and the corresponding chemical shielding δ_ii, is a powerful tool to understand the nature of M=C bonds in Schrock alkylidenes (1).¹ Here, we show how this approach, which combines solid-state NMR spectroscopy and DFT calculations supplemented by localized MO analysis (called NCS analysis), can provide characteristic information about Fischer carbenes (2) and Ru-based carbenes (3) in comparison with Schrock alkylidenes (1) (Figure 1). The highly deshielded ¹³C isotropic chemical shifts observed for Fischer carbenes result from the particularly low-lying π*(M=C) associated with the CO ligand. The same analysis on Ru-based metathesis catalyst show that they are similar to Schrock alkylidenes, albeit with a high-lying Ru=C σ orbital associated with the d⁶ configuration of Ru.

1. Halbert, S.; Copéret, C.; Raynaud, C.; Eisenstein, O. J. Am. Chem. Soc. 2016, 138, 2261.