Högspin

Högspin, often translated as high-spin in English, is a term used in inorganic and solid-state chemistry to describe the electronic state of a transition-metal center when several unpaired electrons are retained. In a crystal-field or ligand-field environment, the five d-orbitals split into a lower-energy set (t2g) and a higher-energy set (eg). Whether electrons pair in the lower set or occupy higher-energy orbitals depends on the ligand-field splitting parameter Δoct and the electron-pairing energy P. When Δoct is smaller than P, electrons tend to maximize unpaired spins, producing a högspin configuration. If Δoct exceeds P, electrons pair in the lower set, yielding a low-spin state.

High-spin states are common for many first-row transition metals bound to weak-field ligands such as water, ammonia, or halides. For example, Fe(II) and Mn(II) complexes with H2O or halides often exist as högspin species (high-spin d6 and d5, respectively). In contrast, strong-field ligands like cyanide or carbon monoxide can stabilize low-spin complexes, such as [Fe(CN)6]3− (low-spin Fe(III)) or [Co(CN)6]3− (low-spin Co(III)).

Properties of högspin species include larger magnetic moments due to unpaired electrons and characteristic colors in UV–visible spectra. Some systems also exhibit spin crossover, a reversible switch between högspin and low-spin states triggered by temperature, pressure, or light. Spin crossover materials are of interest for molecular magnets, sensors, and potential data-storage applications.