Carbon–Carbon Bond Formation in a Weak Ligand Field: Leveraging Open‐Shell First‐Row Transition‐Metal Catalysts

Unique features of earth‐abundant transition‐metal catalysts are reviewed in the context of catalytic carbon–carbon bond‐forming reactions. Aryl‐substituted bis(imino)pyridine iron and cobalt dihalide compounds, when activated with alkyl aluminum reagents, form highly active catalysts for the polymerization of ethylene. Open‐shell iron and cobalt alkyl complexes have been synthesized that serve as single‐component olefin polymerization catalysts. Reduced bis(imino)pyridine iron and cobalt dinitrogen compounds have also been discovered that promote the unique [2+2] cycloaddition of unactivated terminal alkenes. Studies of the electronic structure support open‐shell intermediates, a deviation from traditional strong‐field organometallic compounds that promote catalytic C−C bond formation. Homogeneous catalysis has transformed approaches to carbon–carbon bond formation. This Minireview focuses on unusual iron and cobalt complexes that promote olefin polymerization and cycloaddition reactions.