π-Bond Character in Metal-Alkyl Compounds for C-H Activation: How, When, and Why?

C-H bond activation via c-bond metathesis is typically observed with transition-metal alkyl compounds in d or d fn electron configurations, e.g., biscyclopentadienyl metal alkyls. Related C-H activation processes are also observed for transition metal alkyls with higher d-electron counts, such as W(II), Fe(II), or Ir(III). A sigma-bond metathesis mechanism has been proposed in all cases with a preference for an oxidative addition-reductive elimination pathway for Ir(III). Herein we show that, regardless of the exact mechanism, C-H activation with all of these compounds is associated with pi-character of the M-C bond, according to a detailed analysis of the C-13 NMR chemical shift tensor of the alpha-carbon. pi-Character is also a requirement for olefin insertion, indicating its similarity to sigma-bond metathesis. This observation explains the H-2 response observed in d(0) olefin polymerization catalysts and underlines that sigma-bond metathesis, olefin insertion, and olefin metathesis are in fact isolobal reactions.