t2g4

t2g4 (t2g^4) is a shorthand used in crystal-field theory and ligand-field theory to denote a specific electron distribution among the five d orbitals of a transition-metal ion in an octahedral coordination environment. In this notation, the t2g set consists of the dxy, dxz, and dyz orbitals and is lower in energy than the eg set (dz2 and dx2−y2) under octahedral splitting. The superscript 4 indicates that four electrons occupy the t2g manifold, while the eg set is unoccupied or contains the remaining electrons if the total d-electron count is higher than four.

In practice, t2g^4 commonly describes a d4 metal ion in a strong-field, low-spin octahedral complex, where all four electrons pair within the t2g set (t2g^4 eg^0), yielding a diamagnetic state with low spin. The exact spin and magnetic properties still depend on pairing energy and the overall electron count, but t2g^4 is typically associated with a low-spin configuration for d4. Spectroscopically, transitions from t2g to eg are allowed in many octahedral d-d systems, so t2g^4 configurations contribute characteristic visible absorptions when available.

The notation is also used more broadly to discuss electron occupancy in t2g bands in solid-state chemistry and materials science, especially for transition-metal oxides where itinerant or localized t2g electrons influence conductivity and magnetism.

See also: crystal field theory, ligand field theory, octahedral coordination, Tanabe–Sugano diagrams.