Lowspin

Low-spin is a term used in inorganic chemistry and solid-state chemistry to describe a class of electronic configurations in transition-metal complexes and related materials where electrons pair in the lower-energy orbitals before occupying higher-energy ones. In octahedral or related coordination geometries, low-spin states arise when the crystal field splitting energy, Δo, is larger than the pairing energy, P. Under these conditions, electrons occupy the t2g set with pairing rather than entering the eg set, resulting in fewer unpaired electrons than in high-spin counterparts.

The magnetic and spectroscopic properties of low-spin complexes reflect this electron pairing. They are often diamagnetic or only weakly paramagnetic, and their colors and reactivity can differ from high-spin species. Low-spin behavior is typically associated with strong-field ligands, such as cyanide, carbon monoxide, and certain phosphines, and with metal centers including iron(II), cobalt(III), ruthenium(II), and nickel(II), among others. The precise spin state influences ligand binding kinetics, redox properties, and geometric preferences.

Two common contexts are highlighted. First, many well-characterized octahedral complexes with strong-field ligands are low-spin, such as ferrocyanide, [Fe(CN)6]4−, where iron(II) is low-spin and diamagnetic. Cobalt(III) ammine complexes and several ruthenium(II) carbonyl or cyanide systems are also typical examples. Second, spin-crossover materials can switch between low-spin and high-spin states in response to temperature, pressure, or light, often involving iron(II) centers with suitable ligands; these materials can exhibit bistability and potential applications in sensing or data storage.

See also: crystal field theory, ligand field theory, Tanabe–Sugano diagrams, spin state.