lowvalency

Lowvalency, or low-valent, is a term used in inorganic and organometallic chemistry to describe chemical species in oxidation states that are unusually low for the element involved. In transition-metal chemistry these include metals in oxidation states such as Fe(0), Ni(0), Pd(0), and Cu(I). Low-valent centers are typically highly reactive and can activate small molecules such as H2, CO, and N2, enabling catalytic cycles that involve oxidative addition and reductive elimination. In main-group chemistry, low-valent compounds such as aluminum(I), gallium(I), and certain silicon- or boron-containing species have been stabilized by specialized ligands or multicenter bonding frameworks.

Stabilizing low-valent centers relies on bulky, strongly donating ligands—such as N-heterocyclic carbenes or bulky phosphines—multicenter bonding, and frameworks that suppress aggregation and oxidation. Many low-valent species are prepared by reducing higher-valent precursors under inert conditions, often in the presence of coordinating environments that support the low oxidation state.

Reactivity and applications: low-valent metal centers enable activation of dihydrogen, carbon monoxide, nitrogen, and other small molecules. They are important in homogeneous catalysis, including cross-coupling, hydrofunctionalization, and hydrogenation, and in the study of bond-activation chemistry and the synthesis of unusual main-group compounds. Challenges include air- and moisture-sensitivity, difficulties in isolating stable samples, and sometimes ambiguous oxidation-state assignments due to non-innocent ligands or metal–metal bonding. Ongoing research aims to expand robustness, understanding of mechanisms, and practical applications in synthesis and energy-related chemistry.