Jahn-Teller Distortion in Octahedral Metal Ions

Jahn-Teller Effect

• A non-linear molecule in a degenerate electronic state will distort to remove degeneracy
• Observed in octahedral metal ions with specific electron configurations:
  • d1, d2, d4 (HS or LS), d5 (LS), d6 (HS), d7 (HS or LS), and d9
• Effects are most obvious when the eg* level is unevenly occupied (e.g., HS d4, LS d7, and d9)

Structural Consequences of Jahn-Teller Effect

• With bidentate ligands:
  • Often cannot form conventional tris-chelate complexes
  • Ligand backbone cannot accommodate axial elongation at the metal
• With monodentate ligands:
  • Substantial axial elongation
  • May lose one or both axial ligands

Ligand-Field Stabilization Energies (LFSE)

• LFSEs are greater for octahedral than tetrahedral complexes
• In octahedral complexes, LFSE is greater due to the splitting of degenerate energy levels

Jahn-Teller Distortion

• In Cu2+, there are two ways to arrange eg* electrons, leading to elongation of bonds:
  • (dz2)2(dx2-y2)1: elongation on the z-axis
  • (dz2)1(dx2-y2)2: elongation in the xy plane
• Elongation on the z-axis is favored due to symmetry arguments
• Net result: bonds on the z-axis elongate, and bonds in the xy plane compress

Other Configurations

• Jahn-Teller distortions are also expected for:
  • Tetrahedral metal ions in the ground state: d1, d3, d4, d8, and d9
  • Other configurations may also exhibit Jahn-Teller distortions

Square Planar Complexes

• Rh(I) and Ir(I) are often square planar with p acceptor ligands
• May expand their coordination sphere (important in organometallic catalysis)
• Planar/octahedral equilibria can be finely balanced, leading to anomalous magnetic behavior
• Examples: Lifschitz salts, [Ni(L-L)2]X2 (sq. pl) or [Ni(L-L)2X2] (oct.)