lowvalence

Low valence, or low-valence, is a term used in inorganic and organometallic chemistry to describe chemical species whose oxidation state is lower than typical for the element in a given context. It often refers to metal centers in oxidation states near zero or negative, stabilized by supporting ligands. These species tend to be highly reactive and can be easily oxidized or prone to disproportionation unless protected.

In transition-metal chemistry, common low-valent states include Ni(0), Pd(0), Fe(0), and Cr(0), as seen in carbonyl complexes such as Ni(CO)4, Fe2(CO)9, or Cr(CO)6, and in donor-stabilized complexes like Pd(PPh3)4. These low-valent species are important because they can undergo facile oxidative addition and participate in catalytic cycles for cross-coupling, hydrosilylation, and hydrofunctionalization reactions.

Stabilizing strategies for low-valent metals include bulky, soft ligands (such as bulky phosphines and N-heterocyclic carbenes), low coordination numbers, and strongly donating ligands that protect the metal center from air and moisture. In some cases, metal–metal bonding or cluster formation can also support low-valent centers. Handling typically requires inert atmosphere and anhydrous solvents.

Beyond catalysis, low-valent species are studied for fundamental bond activation, such as binding dihydrogen, carbon monoxide, or dinitrogen, and for materials science applications including magnetic materials and redox-active clusters. The term is used across main group and transition metal chemistry, with ongoing research into expanding the stability and reactivity of low-valent centers and their utility in sustainable chemistry and energy-related applications.