The ‘g’ subscript is used for complexes with a center of symmetry, which octahedral and square planar complexes possess. Tetrahedral complexes lack symmetry, so the ‘g’ subscript is not used in their energy level designations.
Why is the ‘g’ subscript used for octahedral and square planar complexes but not for tetrahedral complexes in terms of energy levels?
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The ‘g’ subscript is used for octahedral and square planar complexes to denote the ground state electronic configuration, where electrons occupy the lower energy orbitals. In these geometries, the ground state often corresponds to a low-spin configuration. In contrast, for tetrahedral complexes, the ground state typically corresponds to a high-spin configuration where electrons occupy higher energy orbitals. As the ‘g’ subscript is associated with the ground state, and tetrahedral complexes commonly adopt high-spin configurations, it is not used for them. The energy level ordering and the resulting electronic configurations differ, leading to this nomenclature distinction.