In isolation, the d orbitals of a metal atom/ion are degenerate due to spherical symmetry. However, the presence of ligands, forming an asymmetrical negative field, leads to the lifting of d orbital degeneracy through splitting.
Why are the d orbitals of an isolated gaseous metal atom/ion degenerate, and how does this degeneracy change in the presence of ligands?
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The d orbitals of an isolated gaseous metal atom/ion are degenerate because they experience a spherically symmetrical field of negative charges. This symmetry ensures that the five d orbitals have the same energy. In the presence of ligands in a coordination complex, the negative field becomes asymmetrical due to ligand electron-metal electron repulsions. This asymmetry lifts the degeneracy of the d orbitals, leading to their splitting into higher-energy eg and lower-energy t₂g sets. This splitting phenomenon, known as crystal field splitting, is a consequence of ligand-induced distortion in the electron distribution around the metal, resulting in distinct energy levels for the d orbitals.