Photochemical reactions, transition-metal complexes

The characterization of electronic excited states has attracted much attention in connection with photochemistry. For example, transition metal complexes are characterized by a variety of absorption spectra in the visible and ultraviolet (UV) regions. The absorption spectra essentially give us information about the electronic excited states corresponding to dipole-allowed transitions due to their high symmetries, while some of the data in crystalline fields indicate the existence of several excited states to which dipole transitions are forbidden in the absence of perturbation. Most photochemical reactions of metal complexes, which are occasionally important as homogeneous photocatalytic reactions, involve both allowed and forbidden excited states. Thus, the systematic understanding of the nature of these excited states is essential in designing photochemical reactions. 

In order to understand the photochemical reactions of metal complexes at the molecular level, it is necessary to know both the number and the energy levels of the spectroscopic states of the complex. The first step in developing a state model is to know the coordination number and structure of the complex about the metal center. For complexes of the lanthanide and actinide ions the coordination number is commonly 8 or 9, but for transition metal complexes a coordination number of 6 is that most frequently observed. 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Transition metal complexes as mediators in photochemical and chemiluminescence reactions. V. Balzani and F. Bolleta, Comments Inorg. Chem., 1983, 2, 211-226 (33). 

Photochemical Reactions Between Transition Metal Complexes and Gases at High Pressures... 

This account summarizes our own results and the reports of other authors regarding the photochemical reactions between transition metal complexes and gases at high pressures. The reactions usually take place in a liquid solvent between dissolved substrates, metal complexes, and dissolved gases which are in equilibrium with a gas phase reservoir. 

MIRBACH Transition Metal Complex Photochemical Reactions... 

Back electron transfer takes place from the electrogenerated reduc-tant to the oxidant near the electrode surface. At a sufficient potential difference this annihilation leads to the formation of excited ( ) products which may emit light (eel) or react "photochemical ly" without light (1,16). Redox pairs of limited stability can be investigated by ac electrolysis. The frequency of the ac current must be adjusted to the lifetime of the more labile redox partner. Many organic compounds have been shown to undergo eel (17-19). Much less is known about transition metal complexes despite the fact that they participate in fljjany redox reactions. 

One possible strategy in the development of low-overpotential methods for the electroreduction of C02 is to employ a catalyst in solution in the electrochemical cell, A few systems are known that employ homogeneous catalysts and these are based primarily on transition metal complexes. A particularly efficient catalyst is (Bipy)Re[CO]3Cl, where Bipy is 2,2 bipyridine, which was first reported as such by Hawecker et al. in 1983. In fact, this first report concerned the photochemical reduction of C02 to CO. However, they reasoned correctly that the complex should also be capable of catalysing the electrochemical reduction reaction. In 1984, the same authors reported that (Bipy)Re[C013CI catalysed the reduction of C02 to CO in DMF/water/ tetraalkylammonium chloride or perchlorate with an average current efficiency of >90% at —1.25 V vs. NHE (c. —1.5V vs. SCE). The product analysis was performed by gas chromatography and 13C nmr and showed no other products. 

Because of the fast nonradiative deactivation of low lying energy states of transition metal complexes, the activation energy for the reactions that may occur from these states must be zero to enable them to compete effectively. For transition metal complexes both 4T2S and aEs states can be photochemically active but may follow different chemical pathways. 

Most of the reported photochemical reactions of lanthanide complexes involve some type of redox behavior.147 Photolysis (254—405 nm) of Eu2+ in acidic aqueous solution, for example, results in photooxidation of the metal and generation of H2 (equation 50).148 While the excited states responsible for this reaction nominally arise from 4/ -+ 5d transitions localized on the metal, strong mixing of the 5rf-orbital with ligand orbitals endows these states with appreciable CTTL character. Photoreduction of aquated Eu3+ can also be driven with UV ( 254 nm) light149 and forms the... 

The complexation of coordination compounds may make it possible to control their photochemical behaviour via the structure of the supramolecular species formed. For instance, the binding of cobalt(m) hexacyanide by macrocyclic polyammonium receptors markedly affects their photoaquation quantum yield in a structure-dependent manner [8.73-8.77]. It thus appears possible to orient the photosubstitution reactions of transition-metal complexes by using appropriate receptor molecules. Such effects may be general, applying to complex cations as well as to complex anions [2.114]. 

Photochemical reactions of transition metal complexes that contain unsaturated chelates fall into three categories 1) fragmentation of the ligand to yield two reactive functionalities 2) elimination of the ligand to generate two reactive sites at the metal 3) chelate localized excited states can function as photoreceptors to promote photodissociation of other metal-ligand bonds in the complex. These processes can be used as an entry to new reactive intermediates and catalysts. 

Unsaturated chelates, in transition metal complexes, photochemical reactions, 177—191 ... 

Abstract The photochemical properties of transition metal complexes, such as those of iridium(III) or ruthenium(II), can be exploited in various ways to generate charge-separated (CS) states, in relation to the mimicry of the natural photosynthetic reaction centres, or to set multicomponent compounds or assemblies in motion. The first part of the present chapter summarizes the work carried out in our groups (Bologna and Strasbourg) in recent years with iridium(III)-terpy complexes (terpy 2,2,6,6"-terpyridine). The synthesis of multicomponent iridium(III) complexes in reasonable yields has been... 

A wide variety of compounds are known which contain at least one transition metal-tin bond. These derivatives undergo different types of reactions, such as substitution of ligands at the tin or the metal center, photochemical reactions and so on. Selected tin derivatives of such transition metal complexes are shown in Table 11. 

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