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

C–H bond activation via σ-bond
metathesis is typically
observed with transition-metal alkyl compounds in d0 or
d0fn 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 σ-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 π-character of the M–C bond, according
to a detailed analysis of the 13C NMR chemical shift tensor
of the α-carbon. π-Character is also a requirement for
olefin insertion, indicating its similarity to σ-bond metathesis.
This observation explains the H2 response observed in d0 olefin polymerization catalysts and underlines that σ-bond
metathesis, olefin insertion, and olefin metathesis are in fact isolobal
reactions.