Sigma complex

Transition metal alkane complexes (e.g., a methane complex) that bind solely through the C–H bond are also known but structurally characterized examples are rare, as C–H σ-bonds are generally poor electron donors, and, in many cases, the weakened C–H bond cleaves completely (C–H oxidative addition) to form a complex of type M(R)(H).

As such, sigma complexes are generally assumed to be intermediates prior to full oxidative addition.

[3] The Wheland complex is an intermediate in the electrophilic substitution reaction on an aromatic compound.

In these, a C-H-σ bond from one of the ligands interacts with the coordinatively unsaturated metal center, forming a chelate complex.

Structurally characterized examples are rare, as C-H-σ bonds generally act as weak electron donors.