THE PHOTOISOMERIZATION REACTIONS OF THE CLUSTER [OS-3(CO)(9)(MU(3)-ETA(2)ETA(2)ETA(2)-C6H6)] AND SOME OF ITS DERIVATIVES

The highly efficient photoinduced isomerisation of [Os-3(CO)(9)(mu(3)- eta(2):eta(2):eta(2)-C6H6)] (1) to the previously characterised mu(3)- benzyne complex [H2Os3(CO)(9)(mu(3)-eta(2):eta(1):eta(1)-C6H4)] (2) is described. We find that for the more electron-rich clusters -n)(PPh(3 ))(n)(mu(3)-eta(2):eta(2):eta(2)-C6H6)](n = 1 or 2) the efficiency of the photolytic process is improved. We suggest that photolysis generat es a Vacant site at osmium (mimicking the inherent coordinative unsatu ration of a metal complex) and that this transformation offers a compe lling model for the interconversion of associatively and dissociativel y chemisorbed benzene, Moreover, we can fine-tune our molecular ''work function'' (really the separation of HOMO/LUMO energies) by protonati on or by phosphine substitution for CO at the metal, Thus, the cation [HOs3(CO)(9)(mu(3)-eta(2):eta(2):eta(2)-C6H6)](+) does not undergo int ernal metalation, while photoisomerisation of the electron-rich phosph ine derivatives proceeds more readily than for 1. We find that these p hotolytic reactions are easily monitored by IR spectroscopy for sample s suspended in polymethylmethacrylate films under ambient conditions. This method permits ''matrix isolation'' experiments under ideal condi tions, Finally, we speculate that the mechanism of interconversion alm ost certainly involves the symmetric cleavage of a Os-Os bond (sigma-s igma transition) to produce an extremely reactive diradical intermedi ate.