Photochemistry of Alkylidene and Vinylidene Complexes

These two different sets of complexes are discussed together because of their obvious structural resemblance. 

For the carbene complexes, only the M(CO)5(alkylidene) (M = Cr, Mo, or W) class has been studied extensively, allowing a comprehensive photochemical picture to be assembled only for that class. 

A one-electron energy level diagram appropriate for the compounds in this series is shown in Fig. 4 and illustrates both and ir[lbi(Tr) - 

A number of results now indicate that irradiation of these complexes can give CO loss but that CO loss does not occur from the MLCT state. The higher energy LF states are responsible for elimination of CO, but the quantum efficiency is poor with 4 = 0.01-0.02. The latter contrasts with the high-effidency CO loss from the parent hexacarbonyls (4 = 0.7-1.0) (167) and is apparently a reflection of rapid internal conversion from the active LF states to the inactive MLCT state. For example, photosubstitution of CH3CN for CO in W(CO)5 C(OMe)Ph was shown to occur cleanly and quantitatively to yield the cis-substituted product shown 

Earlier results also indicated that irradiation of this class of compounds gave CO loss. Thus Fischer and Fischer (168) noted in 1974 that irradiation of Cr- and W-alkylidene complexes in the presence of a series of tertiary phosphines led to CO loss and formation of a mixture of cis- and franj-M(CO)4(PR3)(carbene) products [Eq. (134)]. Low-temperature 

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