Photosubstitutions of mononuclear complexes

Photochemistry has long been a synthesis tool of organometallic chemists given that electronic excitation can surmount enthalpy barriers which might otherwise require undesirably high temperatures. One such example is the common synthetic route to tungsten pentacarbonyl derivatives W(CO)5L indicated by eqs. 2 and 3  

How strong are these solvent interactions with unsaturated fragments such as Cr(CO)s This question has been addressed by photoacoustical calorimetric measurements [29,30]. As expected, traditional Lewis bases coordinate more strongly than do the alkanes but the alkane chromium bond is apparently worth about 10 kcal mol L Time resolved IR spectral experiments have demonstrated an interaction of similar magnitude between W(CO)5 and alkanes in the gas phase [31]. Notably, even xenon has been shown in both solution [32] and gas phase [33] experiments to bind with a dissociation energy of 8 kcal mol .  

As noted above, one of the first indications that photogenerated unsaturated metal carbonyls such as Cr(CO)5 were indeed solvent coordinated was the demonstration in flash photolysis experiments that the rates of the back reactions with CO as well as reactions with other other ligands are markedly dependent on the nature of the solvent medium. For example the second order rate constant k2 for eq. 5 was reported to be 3.6 x 10 M s in cyclohexane[34] and 3 x 10 M s in perfluoro-methylcyclohexane [25]. The reaction kinetics are second order, for the large part, the substitution mechanisms of these intermediates reacting with CO or other substrates are not yet fully elucidated. However, recent kinetics studies by Dobson and coworkers [35] of the reaction of Cr(CO)5S with S = n-heptane or chlorobenzene with 1-hexene or piperidine as a trapping agent have led to the conclusion that the substitution reactions occurred via competitive dissociative and interchange pathways. Complementary studies 

Dinuclear carbonyls have two key features which differentiate them from mononuclear analogs. The first is the more obvious, i.e., the presence of a metal metal bond and the role of the ji-o and transitions which 

In analogy to the mononuclear carbonyls, higher energy excitation can lead to CO dissociation to give unsaturated intermediates (eq. 7) [37,39]. 

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