The σ-CAM Mechanism: σ Complexes as the Basis of σ-Bond Metathesis at Late-Transition-Metal Centers

Complexes in which a σ‐HE bond (E=H, B, Si, C) acts as a two‐electron donor to the metal center are called σ complexes. Clues that it is possible to interconvert σ ligands without a change in oxidation state derive from CH activation reactions effecting isotope exchange and from dynamic rearrangements of σ complexes (see Frontispiece). Through these pathways, metathesis of ME bonds can occur at late transition metals. We call this process σ‐complex‐assisted metathesis, or σ‐CAM, which is distinct from the familiar σ‐bond metathesis (typical for d0 metals and requiring no intermediate) and from oxidative‐reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving σ complexes). There are examples of σ‐CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation. Dynamic processes provide the clue that it is possible to interconvert σ ligands in complexes of the form [ME′(η2‐E‐H)] (E, E′=H, B, Si, C) without a change in oxidation state (see scheme). This σ‐complex‐assisted metathesis (σ‐CAM) is distinct from the familiar σ‐bond metathesis and from oxidative addition/reductive elimination mechanisms. σ‐CAM mechanisms are significant in catalytic and stoichiometric reactions.