Photochemical reactions, of metal

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

The dominant photochemical reaction of metal carbonyl compounds is loss of carbon monoxide, which is usually followed by substitution of another ligand to replace the expelled carbon monoxide. 

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

Photochemical Reactions of Metal Carbonyls with Dihydrogen... 

There are very many known photochemical reactions of metal carbonyls two classic examples are the substitution of a CO group by a donor ligand, for example, a... 

Studies of photochemical reactions of metal complexes and organometallic compounds in solid phase are still far from complete since various difficulties have been encountered in elucidation of their reaction mechanisms. There are a number of factors which influence to a greater or lesser degree the overall spectra of reaction products. Ordinary transmission (absorption) spectroscopic means are generally inconvenient for quantitative characterization of the products since the reactions often take place only in the thin surface layer through which incident light can be transmitted, while the bulk interior remains unreacted. 

This report summarizes conventional methods for UV irradiation of air sensitive organometallic compounds at ambient or subambient temperatures. Of the irradiation sources available (l ) the medium pressure Hanovia 450 W arc lamp systems (2) are of moderate price, reliable, and versatile in our experience. Caution Powerful arc lamps can cause eye damage or blindness within seconds and UV protective goggles (available from most scientific supply houses) must be worn. Never look directly at the radiation source. For safety of other workers lamps should be enclosed in a vented box with baffles. If Pyrex transmits enough UV radiation for an efficient reaction, as for photochemical reactions of metal-metal bonded complexes (3), then conventional Schlenkware can be used for photolysis and no special glassware is needed. Since a 2 mm thick wall of Pyrex transmits only 10% of the UV light at 300 nm, UV transparent quartz reaction vessels are often needed for photoreactions of mononuclear organometallic complexes. 

As outlined in the Introduction, the aim of this chapter is to demonstrate what effect pressure can have on photochemical reactions of metal complexes in solution. The overall photochemical reaction can be accelerated or decelerated by hydrostatic pressure, which results from a combination of the effect of pressure on the photophysical and photochemical processes involved. In many cases, the photochemical processes exhibit a significantly stronger dependence on pressure than the photophysical processes. Thus the quantum yield of such reactions can in general be expected to exhibit a characteristic dependence on pressure, which on the one hand can be used to pressure tune the process, and on the other hand can be used to gain insight into the mechanisms of ES species. Thus the pressure variable adds a further dimension to the investigation of photochemical processes and assists the clarification of intimate reaction mechanisms. 

Chemical and photochemical reactions of metal carbonyl cluster compounds on solid surfaces [analysed by EXAFS]... 

The syntheses of new metal complexes of DTTT 60 have been reported. They are prepared in 80% yield by the photochemical reactions of metal complexes M(CO)6 (M = Cr, Mo, W), Re(CO)sBr, and Mn(CO)3Cp with DTTT 157 (Equation 29) <2003POL1689>. 

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. 

The structural consequences of bonding for carbene complexes are discussed. Photochemical reactions of metal alkyl complexes result in very reactive intermediates that can be trapped and used preparatively. Matrix isolation studies of organometallic intermediates are considered in detail. Transition metals in organic synthesis for the year 1982 are reviewed extensively and a further article concerns a-(1,n)-alkanediy1 complexes. ... 

Reviews dealing with the photochemical reactions of metal carbonyls and the photochemistry of metal carbonyls in matrices have appeared. A third review is concerned with the kinetics and mechanisms of CO substitution in Group VI metal carbonyls. ... 

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. 

Table 6.1. Photochemical Reactions of Metal Carbonyl Complexes having Nitrogen Donor Ligands"...

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