Jahn–Teller distortions

Octahedral complexes of d⁹ and high-spin d⁴ ions are often distorted, e.g. CuF₂ (the solid state structure of which contains octahedrally sited Cu²⁺‏ centres ) and [Cr(H₂O)₆]²⁺‏, so that two metal–ligand bonds (axial) are different lengths from the remaining four (equatorial).

    This is shown in structures 1.1 (elongated octahedron) and 1.2 (compressed octahedron). For a high-spin d⁴ ion, one of the eg orbitals contains one electron while the other is vacant. If the singly occupied orbital is in the d_z2 , most of the electron density in this orbital will be concentrated between the cation and the two ligands on the z axis. Thus, there will be greater electrostatic repulsion associated with these ligands than with the other four and the complex suffers elongation (1.1). Conversely, occupation of the d_x2__y2 orbital would lead to elongation along the x and y axes as in structure 1.2.

    A similar argument can be put forward for the d⁹ configuration in which the two orbitals in the eg set are occupied by one and two electrons respectively. Electron-density measurements confirm that the electronic configuration of the Cr²⁺‏ ion in [Cr)H₂O(₆]²⁺‏ is approximately d_xy¹ d_yz¹ d_xz¹ d_z2¹. confirm that the electronic configuration of the Cr^2‏+ ion in [Cr(H₂O)₆]²⁺‏ is approximately d_xy¹ d_yz¹ d_xz¹ d_z²¹. The corresponding effect when the t_2g set is unequally occupied is expected to be very much smaller since the orbitals are not pointing directly at the ligands; this expectation is usually, but not invariably, confirmed experimentally.  Distortions of this kind are called Jahn–Teller distortions.

(1.1)                                                   (1.2)

   The Jahn–Teller theorem states that any non-linear molecular system in a degenerate electronic state will be unstable and will undergo distortion to form a system of lower symmetry and lower energy, thereby removing the degeneracy.

مواضيع ذات صلة

ايجاد الصيغة التركيبية للمعقدات

معقدات الخارصين

معقدات النحاس

كيمياء ومعقدات بعض عناصر السلسلة الانتقالية الاولى

تعريف المركبات المعقدة

Crystal Field Stabilization Energies

Colors of Transition-Metal Complexes

The Nature of the Ligands

Principal Quantum Number of the Metal

Charge on the Metal Ion

Factors That Affect the Magnitude of Δo

Electronic Structures of Metal Complexes