Download Crystal field theory (II) Octahedral complexes and Jahn

Crystal field theory (II)
Octahedral complexes and Jahn-Teller effect, square planar
complexes, tetrahedral complexes, ligand field splitting
parameters, high spin and low spin complexes
Introduction: The crystal field theory was developed to describe the electronic structure and the magnetic and optical properties of coordination complexes of transition metals. In this theory an electrostatic interaction between the ligands (negative charge) and the (d‐)orbitals of the central atom is assumed. Octahedral complexes and: dx2‐y2 and dz2 orbitals lie along the x, y, z axis while dxy,dxz and dyz orbitals lie in between the axis. Six ligands are considered as approaching along the three axis. So they interact more with dx2‐y2 and dz2 (eg) than with dxy, dxz and dyz orbitals (t2g). The energy of eg becomes higher and that of t2g becomes lower (degeneracy of the five orbitals is lost). The energy difference of t2g and eg is given by Δoct (crystal field splitting parameter). The eg orbitals are raised by +3/5 Δoct while t2g orbitals are lowered by ‐2/5 Δoct relative to the barycentre. High spin and Low Spin Complexes for d4, d5, d6, d7 configurations: if Δoct > pairing energy of electrons, electrons pair and low spin complexes are formed if Δoct < pairing energy high spin complexes are formed Jahn‐Teller effect If the two eg orbitals have different numbers of electrons, this will lead to a further loss of degeneracy of orbitals and distortions take place which called Jahn‐Teller distortion. Jahn‐Teller Effect is observed for high spin d4, low spin d7 and d8 and for d9 complexes; The distortion is either n elongation of two axial bonds and contraction of four equatorial bonds or elongation of four equatorial bonds and contraction of two axial bonds, but generally elongation of two axial bonds and contraction of four equatorial bonds is preferred thermodynamically. Square planar complexes for d8 configurations: Jahn‐Teller distortion leads to tetragonal distortion of the octahedron. The extreme of tetragonal distortion is the loss of the axial ligands, and the formation of a square‐planar complex. As there is no ligand on one z‐axis the d orbitals split further (see textbook). Tetrahedral complexes: The splitting of d orbitals contradicts with those found in the octahedral complex. The donor atoms in tetrahedral coordination do not overlap well with the metal d‐orbitals, so that Δtet is much smaller than Δoct in octahedral complexes with the same ligands (Δtet ≈ 4/9 Δoct), and it is smaller than spin pairing energy. For this reason the tetrahedral complex are usually always high spin complexes. The name of the energy level is (t2, e) instead of (t2g, eg) because Td complexes do not have a center of symmetry. ™ References:
• D. F. Shriver, P. W. Atkins, Inorganic Chemistry Third edition, Oxford University Press, 2001.
• James E. House, Inorganic Chemistry, Illinois Wesleyan University and Illinois State University, 2008 • Catherine E. Houseecroft and Alan G. Sharpe, Inorganic Chemistry second edition, 2005. • MIT, Advanced inorganic and solid state chemistry lectures.
(http://www.youtube.com/watch?v=pZEjVRqe-N4)
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