Charge-transfer band: Difference between revisions
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A charge transfer band |
A '''charge transfer band''' is a light absorption associated with electron transfer within the chromophore. According to IUPAC, this band is "an electronic transition in which a large fraction of an electronic charge is transferred from one region of a molecular entity, called the electron donor, to another, called the electron acceptor."<ref>http://goldbook.iupac.org/C01008.html</ref> |
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==Identification== |
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Charge-transfer bands are identified by<ref name = shriver/> |
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| ⚫ | Charge-transfer bands are identified by a number of characteristics.<ref name = shriver/> An important one is that the band exhibits [[solvatochromism]], i.e. the energy is highly sensitive to the polarity of the solvent, i.e. the color varies with variation in solvent permittivity. This property distinguishes CT bands from the π* ← π transitions. CT absorptions bands are intense, reflecting their allowedness. For [[coordination complex]]es, typical molar absorptivities, ε, are about 50,000 L mol<sup>−1</sup> cm<sup>−1</sup>, three orders of magnitude higher than typical ε of 20 L mol<sup>−1</sup> cm<sup>−1</sup> or lower, for d-d transitions (transition from t<sub>2</sub>g to e<sub>g</sub>). CT transitions are spin-allowed and [[Laporte rule|Laporte]]-allowed. However, d-d transitions are only spin-allowed; they are Laporte-forbidden. CT bands often lie in the ultraviolet or visible portion of the spectrum. |
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*''Color'': The color of CT complexes is indicative of the relative energy balance resulting from the transfer of electronic charge from donor to acceptor. |
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*''Solvatochromism'': In solution, the transition energy and therefore the complex color varies with variation in solvent permittivity, indicating variations in shifts of electron density as a result of the transition. This distinguishes it from the π* ← π transitions on the ligand. |
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==Ligand-to-metal charge transfer== |
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Charge-transfer occurs often in inorganic ligand chemistry involving metals. Depending on the direction of charge transfer they are either classified as ligand-to-metal (LMCT) or metal-to-ligand (MLCT) charge transfer.. |
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===Ligand-to-metal charge transfer=== |
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LMCT complexes arise from transfer of electrons from MO with ligand like character to those with metal like character. This type of transfer is predominant if complexes have ligands with relatively high energy lone pairs (example S or Se) or if the metal has low lying empty orbitals. Many such complexes have metals in high oxidation states (even d<sup>0</sup>). These conditions imply that the acceptor level is available and low in energy. |
LMCT complexes arise from transfer of electrons from MO with ligand like character to those with metal like character. This type of transfer is predominant if complexes have ligands with relatively high energy lone pairs (example S or Se) or if the metal has low lying empty orbitals. Many such complexes have metals in high oxidation states (even d<sup>0</sup>). These conditions imply that the acceptor level is available and low in energy. |
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Another thing to note is that CT bands might also arise from transfer of electrons from nonbonding orbitals of the ligand to the e<sub>g</sub> MO. |
Another thing to note is that CT bands might also arise from transfer of electrons from nonbonding orbitals of the ligand to the e<sub>g</sub> MO. |
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===Trends of LMCT energies=== |
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;Oxidation Number |
;Oxidation Number |
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:+7 MnO<sub>4</sub><sup>-</sup> < TcO<sub>4</sub><sup>-</sup> < ReO<sub>4</sub><sup>-</sup> |
:+7 MnO<sub>4</sub><sup>-</sup> < TcO<sub>4</sub><sup>-</sup> < ReO<sub>4</sub><sup>-</sup> |
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The energies of transitions correlate with the order of the electrochemical series. The metal ions that are most easily reduced correspond to the lowest energy transitions. The above trend is consistent with transfer of electrons from the ligand to the metal, thus resulting in a reduction of metal ions by the ligand. |
The energies of transitions correlate with the order of the electrochemical series. The metal ions that are most easily reduced correspond to the lowest energy transitions. The above trend is consistent with transfer of electrons from the ligand to the metal, thus resulting in a reduction of metal ions by the ligand. |
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Examples |
Examples: |
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# MnO<sub>4</sub><sup>-</sup> : The permanganate ion having tetrahedral geometry is intensely purple due to strong absorption involving charge transfer from MO derived primarily from filled oxygen p orbitals to empty MO derived from manganese(VII). |
# MnO<sub>4</sub><sup>-</sup> : The permanganate ion having tetrahedral geometry is intensely purple due to strong absorption involving charge transfer from MO derived primarily from filled oxygen p orbitals to empty MO derived from manganese(VII). |
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# Fe Oxides: they are red and yellow due to transition from Fe (3d) ← O<sup>2-</sup>(π). |
# Fe Oxides: they are red and yellow due to transition from Fe (3d) ← O<sup>2-</sup>(π). |
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==Metal-to-ligand charge transfer== |
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Metal-to-ligand charge-transfer (MLCT) complexes arise from transfer of electrons from MO with metal like character to those with ligand like character.<ref name = shriver/><ref name = miessler>{{cite book |author=Tarr, Donald A.; Miessler, Gary L. |title=Inorganic chemistry |publisher=Prentice Hall |location=Englewood Cliffs, N.J |year=1991 |pages= |isbn=0-13-465659-8 | edition = 2nd}}</ref> This is most commonly observed in complexes with ligands having low-lying π* orbitals especially aromatic ligands. The transition will occur at low energy if the metal ion has a low oxidation number for its d orbitals will relatively be high in energy. |
Metal-to-ligand charge-transfer (MLCT) complexes arise from transfer of electrons from MO with metal like character to those with ligand like character.<ref name = shriver/><ref name = miessler>{{cite book |author=Tarr, Donald A.; Miessler, Gary L. |title=Inorganic chemistry |publisher=Prentice Hall |location=Englewood Cliffs, N.J |year=1991 |pages= |isbn=0-13-465659-8 | edition = 2nd}}</ref> This is most commonly observed in complexes with ligands having low-lying π* orbitals especially aromatic ligands. The transition will occur at low energy if the metal ion has a low oxidation number for its d orbitals will relatively be high in energy. |
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# Fe(CO)<sub>3</sub>(bipy) |
# Fe(CO)<sub>3</sub>(bipy) |
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===Photoreactivity of MLCT excited states=== |
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The photoreactivity of MLCT complexes result from the nature of the oxidized metal and the reduced ligand. Though the states of traditional MLCT complexes like Ru(bipy)<sub>3</sub><sup>2+</sup> and Re(bipy)(CO)<sub>3</sub>Cl were intrinsically not reactive, several MLCT complexes have been synthesized that are characterized by reactive MLCT states. |
The photoreactivity of MLCT complexes result from the nature of the oxidized metal and the reduced ligand. Though the states of traditional MLCT complexes like Ru(bipy)<sub>3</sub><sup>2+</sup> and Re(bipy)(CO)<sub>3</sub>Cl were intrinsically not reactive, several MLCT complexes have been synthesized that are characterized by reactive MLCT states. |
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Vogler and Kunkely<ref>{{cite journal | author = Vogler, A.; Kunkely, H. | journal = [[Coord. Chem. Rev.]] | year = 2000 | volume = 208 | pages = 321 | doi = 10.1016/S0010-8545(99)00246-5 | title = Photochemistry induced by metal-to-ligand charge transfer excitation}}</ref> considered the MLCT complex to be an isomer of the ground state which contains an oxidized metal and reduced ligand. Thus various reactions like electrophilic attack and radical reactions on the reduced ligand, oxidative addition at the metal center due to the reduced ligand, and outer sphere charge-transfer reactions can be attributed to states arising from MLCT transitions. MLCT states’ reactivity often depends on the oxidation of the metal. Subsequent processes include associative ligand substitution, exciplex formation and cleavage of metal---metal bonds. |
Vogler and Kunkely<ref>{{cite journal | author = Vogler, A.; Kunkely, H. | journal = [[Coord. Chem. Rev.]] | year = 2000 | volume = 208 | pages = 321 | doi = 10.1016/S0010-8545(99)00246-5 | title = Photochemistry induced by metal-to-ligand charge transfer excitation}}</ref> considered the MLCT complex to be an isomer of the ground state which contains an oxidized metal and reduced ligand. Thus various reactions like electrophilic attack and radical reactions on the reduced ligand, oxidative addition at the metal center due to the reduced ligand, and outer sphere charge-transfer reactions can be attributed to states arising from MLCT transitions. MLCT states’ reactivity often depends on the oxidation of the metal. Subsequent processes include associative ligand substitution, exciplex formation and cleavage of metal---metal bonds. |
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==References== |
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===Color of charge-transfer complexes=== |
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{{Reflist}} |
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Many metal complexes are colored due to d-d electronic transitions. Visible light of the correct wavelength is absorbed, promoting a lower d-electron to a higher excited state. This absorption of light causes color. These colors are usually quite faint, though. This is because of two [[selection rule]]s: |
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:The spin rule: Δ S = 0 |
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On promotion, the electron should not experience a change in spin. Electronic transitions which experience a change in spin are said to be ''spin forbidden''. |
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:[[Laporte rule|Laporte's rule]]: Δ l = ± 1 |
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d-d transitions for complexes which have a center of symmetry are forbidden - ''symmetry forbidden'' or ''Laporte forbidden''.<ref>{{cite web | author = Robert J. Lancashire | title = Selection rules for Electronic Spectroscopy | accessdate = 2008-08-30 | url = http://wwwchem.uwimona.edu.jm/courses/selrules.html | publisher = [[University of the West Indies, Mona]]}}</ref> |
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Charge-transfer complexes do not experience d-d transitions. Thus, these rules do not apply and the absorptions are generally very intense. |
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[[Category:Spectroscopy]] |
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For example, the classic example of a charge-transfer complex is that between iodine and starch to form an intense purple color. This has widespread use as a rough screen for counterfeit currency. Unlike most paper, the paper used in US currency is not sized with starch. Thus, formation of this purple color on application of an iodine solution indicates a counterfeit. |
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Revision as of 18:35, 5 March 2013
A charge transfer band is a light absorption associated with electron transfer within the chromophore. According to IUPAC, this band is "an electronic transition in which a large fraction of an electronic charge is transferred from one region of a molecular entity, called the electron donor, to another, called the electron acceptor."[1]
Identification
Charge-transfer bands are identified by a number of characteristics.[2] An important one is that the band exhibits solvatochromism, i.e. the energy is highly sensitive to the polarity of the solvent, i.e. the color varies with variation in solvent permittivity. This property distinguishes CT bands from the π* ← π transitions. CT absorptions bands are intense, reflecting their allowedness. For coordination complexes, typical molar absorptivities, ε, are about 50,000 L mol−1 cm−1, three orders of magnitude higher than typical ε of 20 L mol−1 cm−1 or lower, for d-d transitions (transition from t2g to eg). CT transitions are spin-allowed and Laporte-allowed. However, d-d transitions are only spin-allowed; they are Laporte-forbidden. CT bands often lie in the ultraviolet or visible portion of the spectrum.
Ligand-to-metal charge transfer
LMCT complexes arise from transfer of electrons from MO with ligand like character to those with metal like character. This type of transfer is predominant if complexes have ligands with relatively high energy lone pairs (example S or Se) or if the metal has low lying empty orbitals. Many such complexes have metals in high oxidation states (even d0). These conditions imply that the acceptor level is available and low in energy.
Consider a d6 octahedral complex (example IrBr63-). The t2g levels are filled as shown in Figure 1. Consequently an intense absorption is observed around 250 nm corresponding to a transition from ligand σ MO to the empty eg MO. However, in IrBr62- that is a d5 complex two absorptions, one near 600 nm and another near 270 nm, are observed. This is because two transitions are possible, one to t2g (that can now accommodate one more electron) and another to eg. The 600 nm band corresponds to transition to the t2g MO and the 270 nm band to the eg MO.
Figure 1. MO diagram showing ligand to metal charge transfer for a d6 octahedral complex
Another thing to note is that CT bands might also arise from transfer of electrons from nonbonding orbitals of the ligand to the eg MO.
Trends of LMCT energies
- Oxidation Number
- +7 MnO4- < TcO4- < ReO4-
- +6 CrO42- < MoO42- < WO42-
- +5 VO43- < NbO43- < TaO43-
The energies of transitions correlate with the order of the electrochemical series. The metal ions that are most easily reduced correspond to the lowest energy transitions. The above trend is consistent with transfer of electrons from the ligand to the metal, thus resulting in a reduction of metal ions by the ligand.
Examples:
- MnO4- : The permanganate ion having tetrahedral geometry is intensely purple due to strong absorption involving charge transfer from MO derived primarily from filled oxygen p orbitals to empty MO derived from manganese(VII).
- CdS: The color of artist’s pigment cadmium yellow is due to transition from Cd2+ (5s) ← S2-(π).
- HgS: it is red due to Hg2+ (6s) ← S2-(π) transition.
- Fe Oxides: they are red and yellow due to transition from Fe (3d) ← O2-(π).
Metal-to-ligand charge transfer
Metal-to-ligand charge-transfer (MLCT) complexes arise from transfer of electrons from MO with metal like character to those with ligand like character.[2][3] This is most commonly observed in complexes with ligands having low-lying π* orbitals especially aromatic ligands. The transition will occur at low energy if the metal ion has a low oxidation number for its d orbitals will relatively be high in energy.
Examples of such ligands taking part in MLCT include 2,2’-bipyridine (bipy), 1,10-phenanthroline (phen), CO, CN- and SCN-. Examples of these complexes include:
- Tris(2,2’-bipyridyl)ruthenium(II) : This orange colored complex is being studied[4] as the excited state resulting from this charge transfer has a lifetime of microseconds and the complex is a versatile photochemical redox reagent.
- W(CO)4(phen)
- Fe(CO)3(bipy)
Photoreactivity of MLCT excited states
The photoreactivity of MLCT complexes result from the nature of the oxidized metal and the reduced ligand. Though the states of traditional MLCT complexes like Ru(bipy)32+ and Re(bipy)(CO)3Cl were intrinsically not reactive, several MLCT complexes have been synthesized that are characterized by reactive MLCT states.
Vogler and Kunkely[5] considered the MLCT complex to be an isomer of the ground state which contains an oxidized metal and reduced ligand. Thus various reactions like electrophilic attack and radical reactions on the reduced ligand, oxidative addition at the metal center due to the reduced ligand, and outer sphere charge-transfer reactions can be attributed to states arising from MLCT transitions. MLCT states’ reactivity often depends on the oxidation of the metal. Subsequent processes include associative ligand substitution, exciplex formation and cleavage of metal---metal bonds.
References
- ^ http://goldbook.iupac.org/C01008.html
- ^ a b Cite error: The named reference
shriverwas invoked but never defined (see the help page). - ^ Tarr, Donald A.; Miessler, Gary L. (1991). Inorganic chemistry (2nd ed.). Englewood Cliffs, N.J: Prentice Hall. ISBN 0-13-465659-8.
{{cite book}}: CS1 maint: multiple names: authors list (link) - ^ Kalyanasundaram, K. (1992). Photochemistry of polypyridine and porphyrin complexes. Boston: Academic Press. ISBN 0-12-394992-0.
- ^ Vogler, A.; Kunkely, H. (2000). "Photochemistry induced by metal-to-ligand charge transfer excitation". Coord. Chem. Rev. 208: 321. doi:10.1016/S0010-8545(99)00246-5.
{{cite journal}}: CS1 maint: multiple names: authors list (link)