t2g5

t2g5 is a label used in crystal field theory and ligand-field chemistry to describe a specific electronic configuration of a transition‑metal ion in an octahedral field. It indicates that five electrons occupy the t2g set of d orbitals (the dxy, dxz, and dyz orbitals) while the eg set (dz^2 and dx^2−y^2) has no electrons in the configuration being considered. In an octahedral field, t2g orbitals are lower in energy than eg, so t2g5 corresponds to a d-electron count of five with a strong-field, low-spin arrangement.

In practice, t2g5 is most commonly encountered for d5 ions when ligands are sufficiently strong to pair electrons within the t2g set, yielding a low-spin state with one unpaired electron (S = 1/2). This leads to a spin-only magnetic moment of about 1.7 Bohr magnetons, though some orbital contribution may be present in certain environments. The presence of a t2g hole (the absence of one electron in t2g) can also influence optical and electronic properties, including color and conductivity in solid‑state contexts.

Examples often cited include ferric complexes such as [Fe(CN)6]3−, where cyanide’s strong field promotes a low-spin d5 configuration with t2g5 eg0. The t2g5 arrangement affects spectroscopy by shaping possible d–d transitions and their intensities, and it can influence reactivity, ligand binding, and magnetic behavior. While t2g5 is most straightforward under ideal octahedral symmetry, the concept remains useful for understanding d‑electron occupancy and related properties in lower-symmetry environments.