Metal carbonyls photosubstitution

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

First, the Oh d6 metal carbonyls undergo photosubstitution by a mechanism which is likely none other than simple dissociation of coordinated CO subsequent to the tig " t geg transition. The resulting M(CO)s species, reaction (11), is spectroscopically detectable by a number of... 

Some other examples of substituted metal carbonyl compounds which show wavelength effects in their photosubstitution reactions are given in Table 2. 

Other than the low-valent organometallic and metal-carbonyl complexes, few photosubstitutions of group VIIA complexes are known. Redox reactions predominate among the photochemical processes of Mn complexes, e.g. ... 

Thus the activation volume AV for the rate constant kp of an individual ES reaction pathway can be evaluated if the pressure dependencies of the photoreaction quantum yield, of intersystem crossing and of the ES lifetime can be separately determined. However, such parameterization becomes considerably more complex if several different excited states are involved or if a fraction of the photosubstitution products are formed from states that are not vibrationally relaxed with respect to the medium. Currently, parameterization of pressure effects on photosubstitutions has been attempted for a limited number of metal complexes. These include certain rhodium(III) and chromium(III) amine complexes and some Group VI metal carbonyls, which will be summarized here. 

Recent examples of preparative applications of photosubstitution reactions of metal carbonyls are collected in Table 5 (see p. 193). Others, less suited to tabular presentation, are discussed below in the section dealing with the particular element. In general, synthetic methods involving initial photochemical formation of the metal carbonyl-THF complex, followed by thermal reaction of this complex with an added substituent, have not been included. 

Because of r-backbonding, metal carbonyls are usually quite stable in their ground states. Irradiation of metal carbonyls, on the other hand, often leads to photosubstitution of CO. Several examples are shown in Figure 19.34. This process can be quite useful from a synthetic point of view when replacement of a single CO ligand is desired. 

Some examples of photosubstitution reactions of metal carbonyls. 

Irradiation of [MLg] (M = Cr, Mo, or W L=aryl isocyanide) in pyridine solutions at 436 nm gives monosubstituted derivatives [MLapy] with quantum yields which decrease in the order [Cr(CNPh) ] (0.23)si[Cr(CNIPh)6] (0.23)>[Mo(CNPh)e] (0.055) >[Mo(CNIPh)e] (0.022) >[W(CNPh)6] (0.011)>[W(CNIPh)6] (0.0003). The lower yields for the heavier metals imply that an associative mechanism is involved in which nucleophilic attack by pyridine on the positively charged metal centre of the activated molecule [M" (CNPh)g ] can occur. This is much less facile in the sterically hindered CNIPh (2,6-di-isopropylphenyl) derivatives. These results are in striking contrast to the dissociative photosubstitution reactions of metal carbonyls. 

The photochemistry of rhenium complexes occupies a prominent position in the photochemistry of transition metal complexes. Along with early preparative studies on photosubstitution of carbonyl species like Re(CO)sX, the preparation of the remarkably stable yellow complex /ac-Re(CO)3(phen)Cl foreshadowed the discovery of the a large class of related luminescent materials by Wrighton and co-workers in the 1970s [ 1 ]. As pointed out by Vogler and Kunkley, the current photochemistry of rhenium complexes is rich, spanning eight oxidation states from formal Re(0) (for example, Re2(CO)io) to formal Re(VII) (for example MeReOs) [2],... 

In the photosubstitution of the complexes h -CjHjMCCO L (L = CO, THF, amine, pyridine, or substituted pyridine) for M = Mn only substitution of L is observed in related complexes where L = PPhj, both carbonyl and phosphine photosubstitution are observed Whereas the quantum yields for L replacement are high for M = Mn no matter the identity of L, photosubstitution of h -C5H5Re(CO)2L (L = pyridine with electron-withdrawing substituent) is inefficient because a metal-... 

Transition metal organometallic complexes like dicarbonyl cyclopentadienyl iron [128], tricarbonyl cyclopentadienyl manganese [129] and iron-arene complexes [130,131] have also been reported as photoinitiators for photochemical crosslinking of cyanate esters. Photosubstitution of carbonyl groups by -OCN during irradiation initiates the reaction in the former case whereas photochemical dissociation of arene triggers it in the latter system. 

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