The triangular clusters Os 3 (CO) 1 0 (α-diimine) photoisomerize to give the imine-bridged clusters Os 3 (CO) 1 0 (σ-N, μ 2 -N', η 2 -C N'-α-diimine) if the α-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, 1 H-NMR) data with those of compounds reported in the literature. The quantum yield of the photoreaction decreases with an increase of the steric bulk of the α-diimine. Upon irradiation at low temperature the clusters produce unstable species, which transform into the final products on raising the temperature. The R-PyCa clusters produce a single intermediate, the R-DAB clusters three different ones. Some intermediates are assigned by comparing their IR and UV-vis spectra with those of known species. A reaction mechanism is proposed for the photoisomerization, in which visible excitation causes the homolytic cleavage of an Os-Os bond with formation of a biradical. This biradical undergoes an intramolecular radical coupling reaction of the Os + (CO) 2 (α-diimine - ) and Os(CO) 4 radical sites with formation of an imine-bridged species, which is unstable because of a mismatch 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 depicts a route along which the CO ligands of a cluster may move to compensate for an unbalance in metal-coordination.
