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Photochemical Reactions of Metal Complexes

General Aspects

The absorption of light by a metal complex causes a change of the coordination sphere. Consequently, this physical process can lead to photochemical reactions as well. The d-ortbitals of the central metal can lead to more than two states of spin multiplicity. Hence, the number of excited states is larger for metal complexes than for colored organic compounds. This also results in different reactions that can be initiated by absorption of light. Among the possible reaction types are photodissociation, photosubstitution, geometrical photoisomerization and photooxidationreduction reactions.[1;2]

Photosubsitution Reactions

Absorption of light that induces ππ-transitions of the ligand or d-d-transitions of the metal causes changes of the electron density distribution of the whole complex. With this, the metalligand bond can be weakened and dissociation of this bond can occur. This process is called photodissociation. The product of this reaction is a metal complex an unoccupied coordination position, which is typically unstable[1;2]. An example is the dissociation of CO from a Cr3+ complex:

metaleq1

The free position can be occupied by another molecule. Typical examples for such photosubstitution reactions are the substitution of a ligand with another one. In coordinating solvents the substituting ligand can be a solvent molecule. This reaction type is called photosolvatation. For an ammonia complex of chrome [Cr(NH3)6]3+, one of the ammonia ligands can be exchanged by a water molecule.[1]

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Photoaquation Reactions The most common photochemical reaction in aqueous solutions is the substitution of a ligand by water. An example of these so called photoaquation reactions is the light-induced exchange of water with another water ligand. The exchange of water can be followed by isotope labeled H2O18. The quantum yields (φ) of these reactions are very low (Φ≈ 0.02) and independent from the wavelength. These low quantum yields can be explained with a mechanism involving labilization of a water ligand in the excited state.[3]

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The photoaquation reaction of the chromium chelate complex with ethylenediamine (en) Cr(en)33+ leads to substitution of the coordinated ethylenediamine by an acid (H3O+), followed by protonation of the nitrogen on the free "hinged" ethylenediamine of the chelate. A consecutive second photoaquation follows with exchange of the protonated ethylenediamine by another water molecule. The formation of Cr(en)2(enH)(H2O)4+ can be followed by using a pulsed laser. The complex is formed within 20 ns and a reaction rate constant (k≥108 s−1) for a pseudo first-order reaction was found.[4] Quantum yields of some photoaquation reactions of chromium complexes with different ligands are shown in Table 1.

Table 1: Photoaquation quantum yields.

metalT1

Geometrical Photoisomerizations Substitution of ligands in complexes with different ligands can cause a change of the stereochemistry of the complex. For example, a square-planar cis-complex can isomerize to a trans-complex:

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One ligand dissociates from the complex first and subsequently recoordinates again on a different position. During the time after the complex lost one ligand, the remaining ligands can change position, so that a different isomer results.

Photooxidation-Reduction reactions

When metal complexes are irradiated with light that matches the MLCT or LMCT bands of the complex, the absorption of radiation can lead to intramolecular electron-transfer reactions. The photoredox behavior is induced by the change of redox-potentials during absorption of light. The changing oxidation state of the metal can cause decomposition of the whole complex, as shown below:

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This reaction can be used as chemical actinometer to measure the photonflux.

 

  1. ROUNDHILL, D. M.: Photochemistry and Photophysics of Metal Complexes, 1994, Springer, ISBN 0306446944.
  2. BALZANI, V.; CERONI, P.; JURIS, A.: Photochemistry and Photophysics, 2014, Wiley VCH Verlag GmbH, ISBN 3527334793.
  3. PLANE, R. A.; HUNT, J. P.: Photochemical Exchange of Water between Cr(H2O)63+ andSolvent, Journal of the American Chemical Society, 1957-July, 79 (13), 3343–3346, DOI: 10.1021/ja01570a010.
  4. GEIS, W.; SCHLAEFER, H. L.: Trisethylenediaminechromium(III) complex ion photolysis in acidic aqueous solution, Zeitschrift fuer Physikalische Chemie (Muenchen, Germany), 1969, 65.
  5. WEGNER, E. E.; ADAMSON, A. W.: Photochemistry of Complex Ions. III. Absolute Quantum Yields for the Photolysis of Some Aqueous Chromium(III) Complexes. Chemical Actinometry in the Long Wavelength Visible Region, Journal of the American Chemical Society, 1966-February, 88 (3), 394–404, DOI: 10.1021/ja00955a003.
  6. EDELSON, M. R.; PLANE, R. A.: The Photochemical Aquation of Cr(NH3)+6 3 and Cr(NH3)5H20+3, The Journal of Physical Chemistry, 1959-March, 63 (3), 327–330, DOI: 1021/j150573a002.
  7. CHIANG, A.; ADAMSON, A. W.: Photochemistry of aqueous Cr(CN)63-, The Journal of Physical Chemistry, 1968-October, 72 (11), 3827–3831, DOI: 1021/j100857a022.
  8. SPEES, S. T.; ADAMSON, A. W.: Photochemistry of Complex Ions. II. Photoracemization, Inorganic Chemistry, 1962-August, 1 (3), 531–539, DOI: 1021/ic50003a018.

 

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