Laporte and Spin Selection Rules (PDF)

1. Laporte Selection Rule Statement : Only allowed transitions are those occurring with a change in parity (flip in the sign of one spatial coordinate.) OR During an electronic transition the azimuthal quantum number can change only by  1 ( l = 1) The Laporte selection rule ref...

1. Laporte Selection Rule Statement : Only allowed transitions are those occurring with a change in parity (flip in the sign of one spatial coordinate.) OR During an electronic transition the azimuthal quantum number can change only by  1 ( l = 1) The Laporte selection rule reflects the fact that for light to interact Otto Laporte with a molecule and be absorbed, there should be a change in dipole German American Physicist moment. Practical meaning of the Laporte rule Gerade = symmetric w r t Allowed transitions are those which occur centre of inversion between gerade to ungerade or ungerade to Ungerade = antisymmetric gerade orbitals w r t centre of inversion Allowed g u & u g This rule affects Octahedral and Square planar complexes as they Not allowed (FORBIDDEN) have center of symmetry. g g & u u Tetrahedral complexes do not have t2g eg is forbidden OR center of symmetry: therefore, this According to Laporte selection rule rule does not apply d→d transitions are not allowed ! 1. Laporte Selection Rule  2. Spin Selection Rule Statement : This rule states that transitions that involve a change in spin multiplicity are forbidden. According to this rule, any transition for which  S = 0 is allowed and  S  0 is forbidden Practical significance of the Spin Selection rule During an electronic transition, the electron should not change its spin d5 High spin (e.g [Mn(H2O)6]2+ eg eg h [GS] [ES] [GS] [ES] t2g t2g S = 0 S  0 S  0 Forbidden Allowed Forbidden  Why do we see ‘forbidden’ transitions at all? Relaxation of the selection rules There are three mechanisms that allow ‘forbidden’ electronic transitions to become somewhat ‘allowed’ resulting in some intensity of the color expected. http://www.star.le.ac.uk/~zrw/courses/lect4313_fig22.jpg 1) Vibronic Coupling: During some unsymmetrical vibrations of a molecule there can be a temporary/transient loss of the centre of symmetry. Loss of center of symmetry helps to overcome the Laporte selection rule. Also, time required for an electronic transition to occur (lifetime 10-18 sec) is much less than the time required for a vibration to occur (lifetime 10-13 sec). 2) Mixing of states: The states in a complex are never pure, and so some of the symmetry properties ( g or u) of neighboring states become mixed into those of the states involved in a ‘forbidden’ transition. For example mixing of d (gerade) and p (ungerade) orbitals results in partial breakdown of the Laporte rule 3) Spin orbit coupling: Partial lifting of the spin selection rule is possible when there is coupling of the spin and orbital angular momentum, known as the spin-orbit coupling ( common in heavier transition metals) [Mn(H2O)6]2+ The spectra of complexes of tetrahedral metal ions: As we have seen, a tetrahedron has no center of symmetry, and so orbitals in such symmetry cannot be gerade. Hence the d- levels in a tetrahedral complex are e and t2, with no ‘g’ for gerade. This largely overcomes the Laporte selection rules, so that tetrahedral complexes tend to be more intense in color. Thus, we see that dissolving CoCl2 in water produces a pale pink solution of [Co(H2O)6]2+, but on adding HCl tetrahedral [CoCl4]2- forms, which has a very intense blue color. Cobalt blue was known in China before 1400 BC when it was used for pottery glazes, but it was always a rare pigment because cobalt minerals were scarce. Today, cobalt is still used to colour porcelain, pottery, glass, tiles and enamel jewellery. Its rich blue colour is also known as Sèvres blue and Thénard blue The spectra of octahedral [Co(H2O)6]2+ and tetrahedral [CoCl4]2- ions: [CoCl4]2- Intense d-d bands in the blue tetrahedral complex [CoCl4]2-, as compared with the much weaker band in the pink octahedral complex [Co(H2O)6]2+. This difference arises because the Td complex has no center of symmetry, helping to overcome the g→g Laporte selection rule. [Co(H2O)6]2+  Classification of intensities of electronic transitions Transition type Example Typical values of ε /m2mol-1 Spin forbidden, Laporte forbidden (partly allowed by spin–orbit [Mn(H2O)6]2+ 0.1 coupling) Spin allowed (octahedral complex), Laporte forbidden [Co(H2O)6]2+ 1 - 10 (partly allowed by vibronic coupling and d-p mixing) Spin allowed (tetrahedral complex), [CoCl4]2- 50 - 150 Laporte allowed (but still retain some original character) Spin allowed, Laporte allowed KMnO4 1000 - 106 e.g. charge transfer bands  Molecular orbital picture required to explain charge transfer spectra Includes both  and π (back bonding) bonding t1u KMnO4 Prussian Blue Ferric Thiocyanate Pot. Dichromate  What happens if the absorption of electromagnetic radiation for an octahedral complex falls in the ultraviolet range? Cr(CO)6 Mo(CO)6 Colorless W(CO)6 However, some complexes also show a phenomenon known as Fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. In most cases, the emitted light has a longer wavelength and a lower energy than the absorbed radiation. So, if such complexes are irradiated with UV light, the excited electron will lose some energy and then fall back to ground state emitting (fluorescent) light in the visible range Emission spectra, Black light, UV-A radiation and fluorescence Zn2SiO4 Quinine  Advantages and Disadvantages of Crystal Field Theory Advantages over Valence Bond theory 1. Explains colors of complexes 2. Explains magnetic properties of complexes ( without knowing hybridization) and temperature dependence of magnetic moments. 3. Classifies ligands as weak and strong 4. Explains anomalies in the physical properties of metal complexes 5. Explains distortion in shape observed for some metal complexes Disadvantages or drawbacks 1. Evidences for the presence of covalent bonding ( orbital overlap) in metal complexes have been disregarded. e.g Does not explain why CO although neutral is a very strong ligand 2. Cannot predict shape of complexes (since not based on hybridization) 3. Charge Transfer spectra not explained by CFT alone  Problem Solving Arrange the given metal complexes in the increasing order of intensity of color () shown by them. Justify your order by writing below each the status of the selection rules for these complexes Fe4[Fe(CN)6]3 , [CoBr4]2−, [MnF6]4−, Least intense Most intense Spin Forbidden Spin Allowed Spin Allowed Spin Allowed Laporte Allowed Laporte Forbidden Laporte Laporte Allowed Charge – Transfer Forbidden (Tetrahedral) transition