Photochemical isomerisation, reversible

In Section 5.6.1.1 quantitative results for the partial photochemical quantum yields of the photoreaction of stilbene-1 were given. A photoisomerisation was assumed to be the mechanism as a first step. This proposal is supported by a reaction chromatogram of the photodegradation reaction of this laser dye. The two parts of the diagram (see Fig. 5.58) prove a photo-reversible isomerisation as a first step [190]. This information was used for the evaluation mentioned. 

The seminal report in this area documented the thermally-induced isomerisation of Tp Ir(CO)(>/ -C2H4) (229) to Tp = TrH(CH=CH2)(CO) (289), which proceeds cleanly at 60 °C. This thermodynamic preference is the reverse of that observed for the Cp and Cp analogues, which favour the j/ -alkene complex, as does the photochemically generated Tp RhH(CH=CH2)(CO) (290), which rapidly reverts to Tp Rh( -C2H4)(CO) (148) under ambient conditions. This disparity between the rhodium and iridium 71-alkene complexes has been widely observed. However, while the rhodium hydridovinyls are rarely isolated, the kinetic accessibility of Tp RhH(CH = CH2)(f/ -C2H4) (294) has been established. It was found that on treatment with NCMe or pyridine, Tp Rh( -C2H4)2 (118) cleanly converts to Tp Rh(CH2CH3)(CH = CH2)(L) (L = NCMe (156), py (293)),... 

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