Mechanistic study of the photoisomerization of Os-3(CO)(10)(L) in which L (L = 1,4-di-R-1,4-diazabutadiene (R-DAB) or pyridine-2-carbaldehyde N-R-imine (R-PyCa)) changes its coordination from sigma, sigma-N,N ' into sigma-N,mu(2)-N ', eta(2)-C=N '

The triangular clusters Os-3(CO)(10)(alpha-diimine) photoisomerize to give the imine-bridged clusters Os-3(CO)(10)(sigma-N, mu(2)-N', eta(2)-C=N'-alph a-diimine) if the alpha-diimine has a reactive imine bond as in the case of R-DAB (1,4-di-R-1,4-diazabutadiene) or R-PyCa (pyridine-2-carbaldehyde N-R -imine). The products are identified by comparing their spectroscopic (IR, UV-vis,H-1-NMR) data with those of compounds reported in the literature. Th e quantum yield of the photoreaction decreases with an increase of the ster ic bulk of the alpha-diimine. Upon irradiation at low temperature the clust ers produce unstable species? which transform into the final products on ra ising the temperature. The R-PyCa clusters produce a single intermediate, t he R-DAB clusters three different ones. Some intermediates are assigned by comparing their IR and W-vis spectra with those of known species. A reactio n mechanism is proposed for the photoisomerization, in which visible excita tion causes the homolytic cleavage of an Os-Os bond with formation of a bir adical. This biradical undergoes an intramolecular radical coupling reactio n of the Os+(CO)(2)(alpha-diimine(.-)) and Os-.(CO)(4) radical sites with f ormation of an imine-bridged species, which is unstable because of a mismat ch between the coordination of the two Os atoms. This mismatch is lifted by transfer of a CO ligand from one Os to the other via two CO-bridged (Os-CO -N) intermediates, which are detected at low temperature. This mechanism de picts a route along which the CO ligands of a cluster may move to compensat e for an unbalance in metal-coordination. (C) 1999 Elsevier Science S.A. Al l rights reserved.