highspin

High-spin is a term used in atomic, molecular, and solid-state chemistry and physics to describe electronic states with relatively large total spin. In coordination chemistry, a high-spin complex is a metal complex in which electrons remain unpaired as much as possible because the crystal-field splitting between d-orbitals is small compared with the pairing energy. Consequently, electrons occupy higher-energy orbitals before pairing in lower-energy ones. This is contrasted with low-spin complexes, where strong-field ligands produce a large splitting that favors pairing in the lower set of orbitals.

Implications of high-spin states include typically higher paramagnetic moments, different colors and ligand-field transitions, and distinct reactivity patterns. High-spin complexes are common with first-row transition metals in weak-field ligands such as water, hydroxide, fluoride, and chloride; low-spin complexes are favored with strong-field ligands like cyanide, carbon monoxide, and ethylenediamine. In Fe(II) and Fe(III) chemistry, spin states influence magnetic and spectroscopic properties and help explain observed behavior in various oxides and coordination compounds.

Spin-crossover refers to systems, especially some Fe(II) complexes, that can reversibly switch between high-spin and low-spin states in response to temperature, pressure, or light. This phenomenon is of interest for molecular switches and memory applications.

In nuclear and particle physics, high-spin states describe nuclear excited states with large total angular momentum, typically explored through gamma-ray spectroscopy. The term is used across disciplines to denote states with relatively large spin, affecting magnetic, spectroscopic, and dynamical properties.