‘‘For almost any set of degenerate electronic states associated with a molecular configuration there will exist some symmetry-breaking interaction in which molecular distortion is associated with the removal of the electronic degeneracy.’
Jahn-Teller Distortion
Jahn-Teller effect is a type of geometrical distortion commonly observed among octahedral complexes where the two axial bonds can be shorter or longer than those of the equatorial bonds. This effect is very weak in tetrahedral compounds.
Types of Jahn Teller distortion
Jahn teller distortion can be classified into two categories based upon the observed geometries.
1. 1. Static Jahn Teller Distortion
2. 2.Dynamic Jahn Teller Distortion
1. Static Jahn Teller Distortion
In some molecules, only tetragonal geometry is observed under all conditions i.e., in solid as well as liquid state; at higher as well as lower temperatures. This is called as static Jahn teller distortion. In other words, static Jahn teller distortion is strong as well as permanent. It is usually observed in complexes possessing asymmetrically filled eg orbitals.
Example:
Static Jahn teller distortion is observed in CuF2 lattice.
2. Dynamic Jahn Teller Distortion
In some molecules, complexes can attain both states i.e., compressed state (z-in) as well as elongated (z-out). This is common for molecules where the energy gap between the z-in and z-out states is less as compared to the thermal energy available. This is called dynamic Jahn teller distortion. It is observed only at lower temperatures. Example: K2Pb[Cu(NO2)6] complex shows dynamic Jah teller distortion.  Jahn teller effect in Octahedral complexes The Jahn Teller effect in octahedral complexes is also known as tetragonal distortion. Jahn Teller effect is commonly observed when there is presence of degenerate electronic configuration i.e., there are more than one possible site for electrons to be filled. Whenever a degenerate electronic configuration is possessed by the molecule, it will tend to remove the degeneracy in order to form a lower energy system and attain stability. As a result, octahedral complexes will tend to elongate or compress the metal to ligand bond in this case. The d orbitals in octahedral complexes can be classified into ü eg orbitals (which point directly at the incoming ligands) ü t2g orbitals (which point in between the axis and do not directly point at the incoming ligands) Being directly interacting with the incoming ligands, eg orbitals experience greater interaction with the ligands. As a result, Jahn teller effect is more prominent in these cases. For example, d9 complexes of Cu+2. There are three electrons present in doubly degenerate eg orbitals producing a doubly degenerate electronic state. As a result, there is distortion along z axis which tends to remove the orbital and molecular degeneracy and tends to lower the overall energy. The amount of overlap occurring between metal and ligand determines the Elongation and compression effects. So, the extent of Jahn teller distortion is greatly influenced by type of metal and ligands. As a general rule, the greater is the interaction between metal and ligand orbitals, more are the chances for Jahn Teller distortion to occur. The distortion mostly involves elongation of metal-ligand bond along x – axis but in some cases compression can also occur. It is to be noted that prediction of unstable geometry is made by Jahn teller theorem. However, it does not tell about the direction of distortion. Elongation and compression effects Elongation (z-out) When axial bonds become larger than the equatorial bonds, the complex is said to be exhibiting elongation. Reason Elongation results when d orbital with z component is stabilized while that without z component is destabilized thereby breaking the degeneracy. The reson behind this is that dxy and dx2-y2 have greater overlap with the orbitals of ligands thereby increasing their energy. Being antibonding orbital, dx2-y2 is raised in energy due to elongation. Despite being non bonding , dxy is destabilized due to interactions. Example: Ø Geometry of copper (II) fluoride The structure of copper (II) fluoride exhibiting elongation is represented in the figure below. The two axial bonds are elongated while the four equatorial bonds, shorter in length, are equal. Orbital degeneracy is removed as a result of this distortion making the molecule stable. Compression (z-in) In case of compression, the equatorial bonds are longer as compared to the axial bonds. Reason: Compression results when d orbital without z component is stabilized while that with z component is destabilized thereby breaking the degeneracy. This occurs when the overlap of d orbitals having z component is greater with the ligands thereby raising the energy of the orbitals. Being antibonding orbital, dz2 is raised in energy due to elongation. Despite being nonbonding , dxy and dyz are destabilized due to interactions. Example: Geometry of [Co(NH3)6]2+ Low-spin Co2+ octahedral complex has electronic configuration t2g 6 eg 1. If electron of eg is present in dx2-y2 , ligands approaching along x and y axis will experience more repulsion than those approaching from z axis. So bonds along x and y axis will be weaker resulting in tetragonally flattened geometry along z axis comprising two shorter and four longer bonds. Electronic Configurations of Octahedral Complexes The necessary condition for Jahn teller effect to occur in transition metal is that there should be splitting of d-orbital into t2g and eg orbital and its electronic state depends upon number of d- electron and splitting energy. Low and high spin complexes formed on the basis of splitting energy ΔΔ. Small ΔΔo: High Spin complexes High spin complexes usually formed under the influence of week field ligand. Under the influence of week field ligand the splitting between t2g and eg is comparatively lesser than strong field hence electron will fill the higher energy orbital. Jahn teller effect will not be shown by complexes containing d3, d5, d8, d10 because jahn teller effect results due to unequal occupation of degenerate orbitals. In case of d3, d5, d8, d10 there is no unequally occupied degenerate orbital hence jahn teller effect will not be observed. Jahn teller effect is more prominent in case of eg orbitals singly occupied by the electron hence in case of high spin complexes it will be more prominent in d4 and d9 complexes. Jahn teller effect in high spin d4 & d9 complexes Ø d4 complex Hexaaquachromium (II) complex ion, [Cr(OH2)6] 2+ Electronic configuration: t2g3 eg1 Geometry: High spin d4 hexaaquachromium (II) complex ion show 2 geometries: the first geometry is one in which two Cr-O bonds length are elongated than other four Cr-O bond length while the second geometry is one in which two Cr-O bond length are shorter than other four Cr-O bonds this indicate that Cr d4 high spin complex show both elongation and compression Jahn-Teller distortion. Similar is the case with Cu complex. Elongation distortion In the case of elongation (z-out) distortion of the d4 complex, HOMO and LUMO both are α- frontier molecular orbitals and these d-based α-frontier molecular energy levels are in the following order of ascending energy: dxy < dxz < dyz < dz2 (HOMO) < dx2 y 2 (LUMO). Since LUMO is empty and HOMO (dz2) occupied one electron hence large concentration of electronic density occur between metal and two ligands along the z-axises than between metal and four ligands located in xy-plane .Hence, responsible for elongation jahn teller distortion. Compression distortion: The compression distortion in d4 complexes, show following order of α-frontier molecular energy levels: dxz < dyz < dxy < dx2-y2 (HOMO) < dz2 (LUMO) As LUMO is empty and HOMO contain 1 electron the electronic density is concentrated along xy- plane rather than along z-axises hence show compression of bonds along z-axises. Ø d9 complex Hexaaquacopper (II) complex ion, [Cu(OH2)6] 2+ Electronic configuration: t2g6 eg3 Geometry: d 9 complexes show both Jahn teller compression distortion and elongation distortion similar to that observed in case of d4 complexes but the difference is that here the LUMO and HOMO are β-orbitals. Condition # 1: t2g6 [(dx2-y2)2 (d 2)1] If the dx2-y2 orbital contain pair of electrons, then it would more effectively shield the ligands lying in the xy plane from the metal ion’s charge in comparison to the shielding provided by the single electron present in the dz2 orbital lying along the z axis. In this case, the ligands along z-axis would be more strongly attracted to the central metal ion hence shortened the M-L bond lengths relative to those in the xy-plane.
Condition # 2: t2g6 [(dx2-y2)1 (d 2)2] If the dx 2 -y 2 orbital contain single electron then the shielding provided by that electron would be less in comparison to the shielding provided by two electron present in dz2 orbital and hence the bond length between the metal to the ligand lying in dz2 would be relatively longer than the bond length between metal to the ligand lying in xy-plane. Large ΔΔo: Low Spin complexes Strong field ligands usually causes greater splitting between t2g and eg orbital and hence lead to the formation of low spin complexes. In case of low spin complexes, d3, d6, d8, d10 all the orbitals are equally occupied and hence will not show jahn teller distortion. In both high spin and low spin complexes, electronic configuration of d1, d2, d3, d8, d9, and d10 remain same. d4, d5, d6, and d7 configuration changes in low and high spin complexes. Controversy in Jahn teller distortion Low spin complexes Controversy occur in case of low spin d8 complexes about which there are 2 different opinions of researchers. Ø According to one opinion, low spin d8 systems, like Ni(II) are degenerate in the octahedral environment as under the influence of strong field ligand electron paired up in eg orbital hence eliminated dz2 orbital thus Ni(CN)4 exhibit square planar geometry. As this rule did not follow Hund’s rule hence remain debatable. Ø The second opinion states that Ni(CN)4 exhibit square planar geometry because of the larger Δo value and infinite distortion along z-axis but this distortion is not jahn teller distortion because the system is not at all electronically degenerate in octahedral environment. High spin complexes
High spin complexes e.g NiCl42- are tetrahedral instead of square planar because of small Δo value. While all the high spin and low spin complexes of platinum and palladium are square planar. \The reasons are:
Ø 50% increment in Δo value as we move down the group.
Ø Large size of metal minimizes the ligand repulsion hence favored square planar geometry.
Ø Stability of square planar geometry increases due to interaction between d-orbital of metal and ligand. Due to small size of Ni (II) π-interactions are not possible hence no effective overlap. Jahn teller effect in tetrahedral complexes In tetrahedral complexes orbitals are directed in opposite position to that in octahedral complexes. Jahn teller distortion is significant in case of complexes where the orbitals are oriented along axises through which ligand approaches the central metal which is possible in octahedral complexes. Jahn teller distortion in tetrahedral complexes is generally week because of two reasons: Ø Tetrahedral complexes has no centre of symmetry. Ø In tetrahedral complexes, d-orbitals are oriented in between the axis while ligand approaches along the axis hence, experience minimum repulsion. However, exception is when t2g set is unsymmetrically filled and causes considerable jahn teller distortion in tetrahedral geometry. Conclusions It is important to summarize a few key points about the Jahn-teller effect. In certain cases, the effect is negligible. The magnitude of the Jahn-Teller effect is difficult to estimate using experimental methods. The distortions usually lead to one of the molecule's natural modes of vibration. The theorem does not predict which way the distortion will occur; it just informs us when it will happen. If the degeneracy is lifted by spin-orbit coupling, ligand inequivalence, or an environment-forced ligand displacement, the effect is not observed. There are two types of the Jahn-Teller effect that can be distinguished. In the static JTE, one configuration is preferred, and the molecule is permanently distorted. When two twisted structures are separated by a thin membrane, the molecule flips back and forth between them, resulting in dynamic JTE. The time- averaged positions of the ligands in a transition metal compound with coordination number six and subject to the dynamic Jahn-Teller effect may be similar to a normal octahedron. Spectroscopic studies like UV - VIS, IR, ESPR and Raman Spectroscopy also reveal JTE in various molecules. JTE is applicable to study the effect on structure of small molecules and coordination complexes, furthermore, it is also applicable for solid state problems. References Websites consulted 1. https://en.wikipedia.org/wiki/Jahn%E2%80%93Teller_effect 2. https://www.slideshare.net/MEGHNATH97/jahn-teller-effect https://chem.libretexts.org/ 3. https://www.adichemistry.com/inorganic/cochem/jahnteller/jahn-teller-distortion.html
4. https://www.dalalinstitute.com/wp-content/uploads/Books/A-Textbook-of-Inorganic-Chemistry- Volume-1/ATOICV1-8-7-Jahn-Teller-Effect.pdf 5. http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/S000005CH/P000662/M010466/ET/s0 00005ch-p000662-m010466-et-v1.pdf 6. https://www.reddit.com/r/chemistry/comments/2ljej1/why_is_it_that_with_jahnteller_effect_co mplexes/ 7. https://staffsites.sohag-univ.edu.eg/uploads/1379/1543675231%20-%20lezione29.pdf
8. https://www.quora.com/Why-is-Jahn-Teller-distortion-not-observed-in-tetrahedral-complexes
9. http://alpha.chem.umb.edu/chemistry/ch612/files/Overheads/Topic14_2_Carter_Ch7.pdf
10. https://www.adichemistry.com/inorganic/cochem/jahnteller/jahn-teller-distortion.html Research papers consulted 1. The Jahn–Teller effect: An introduction and current review American Journal of Physics 61, 688 (1993); https://doi.org/10.1119/1.17197 2. Jahn, H. A.; Teller, E. (1937). "Stability of polyatomic molecules in degenerate electronic states. I. Orbital degeneracy". doi:10.1098/rspa.1937.0142 3. Bersuker, I. B. (1963). "Inversion Splitting of Levels in Free Complexes of Transition Metals". Sov. Phys. JETP. 4. O'Brien, M. C. M. (1964). "Dynamic Jahn–Teller Effect in Octahedrally Co-ordinated d9 Ions". Proc. R. Soc doi:10.1098/rspa.1964.0185. 5. Conradie, J. (2019). Jahn-Teller effect in high spin d4 and d9 octahedral metal-complexes. Inorganica Chimica Acta, 486, 193–199. doi:10.1016/j.ica.2018.10.040 6. Öpik, U.; Pryce, M. H. L. (1957). "Studies of the Jahn Teller Effect.1. A Survey of the Static Problem". Proc. R. Soc. A. 238 (1215): 425–447. Bibcode:1957RSPSA.238..425O. doi:10.1098/rspa.1957.0010. 7. Deeth, R. J.; Hitchman, M. A. (1985). "Factors Influencing Jahn–Teller Distortions in Six- Coordinate Copper(II) and Low-Spin Nickel(II) Complexes". Inorg. Chem. 25 (8): 1225– 1233. doi:10.1021/ic00228a031
8. Kaplan, M. D.; Vekhter, B. G. (1995). Cooperative phenomena in Jahn–Teller crystals. New York: Plenum Press. ISBN 978-1-4615-1859-4.
9. Lee, J. H.; Delaney, K. T.; Bousquet, E.; Spaldin, N. A.; Rabe, K. M. (2013). "Strong coupling of Jahn–Teller distortion to oxygen-octahedron rotation and functional properties in epitaxially strained orthorhombic LaMnO3". Phys. Rev. B. 88 (17): 174426. arXiv:1307.3347. Bibcode:2013PhRvB..88q4426L. doi:10.1103/PhysRevB.88.174426.
10. ^Kugel, K. I.; Khomskii, D. I. (1982). "Jahn–Teller Effect and Magnetism – Transition-Metal Compounds". Sov. Phys. Usp. 25 (4): 231–256. doi:10.3367/UFNr.0136.198204c.0621 |
Comments
Post a Comment