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Charge-Transfer

The large energy of photons can induce different electronic excitations in metal complexes. The decreasing transmission at a certain wavelength can be observed as the color of the complexes. Many pigments used industrially are colored due to charge-transfer transitions. In metal complexes, four different electronic transitions can be distinguished:

  • metal charge transfer (MC),
  • metal to ligand charge transfer (MLCT),
  • ligand to metal charge transfer (LMCT) and
  • ligand to ligand charge transfer (LC).

The schematic representation is shown in Figure 1. The metal charge transfer (MC) is an electronic transition between the d-orbitals of the metal ion. Low energy MCs can be expected for the 3rd period transitions metals. The metal to ligand charge transfer (MLCT) is the transition of an electron from an orbital of the metal ion to an orbital of the ligand, more precisely from a d-orbital of the metal ion towards the π-orbital of the ligand. MLCT-Transitions are to be expected, when the metal is easy to oxidize and the ligand is easy to reduce. The ligand to metal charge transfer (LMCT) is the equivalent transfer of an electron from the ligand π-orbital towards the d-orbital of the metal. LMCTs can be expected when the metal is easy to reduce and the ligand is easy to oxidize. The ligand to ligand charge transfer (LC or LLCT) is the transition from a π-orbital of a ligand towards the π-orbital of the ligand. LCs usually occur in aromatic systems with an extended π-system.

 Chargetansfer

 Figure 1: Possible charge transfer transitions shown for an octahedral 3d-complex as an example.

 

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  2. MATTAY, J.; GRIESBECK, A. G.: Photochemical Key Steps in Organic Synthesis. An Experimental Course Book, 1994, Wiley-VCH, ISBN 978–3–5272–9214–1.
  3. Comprehensive Heterocyclic Chemistry, 2009, Elsevier, ISBN 978-0-08-096519-2.
  4. VOLLHARDT, K. P. C.; SCHORE, N. E.: Organic Chemistry, 6th Edition, 2010, W. H. Freeman, ISBN 978-1-4292-0494-1.

 

 

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