PHOTOCHEMISTRY OF THE CLUSTERS OS-3(CO)(10)(L) (L=2,2'-BIPYRIDINE, 2,2'-BIPYRIMIDINE, 2,3-DIPYRID-2-YLPYRAZINE, 2,3-DIPYRID-2-YLBENZOQUINOXALINE) - REVERSIBLE OPENING OF AN OS-OS BOND WITH FORMATION OF A ZWITTERION

The complexes Os-3(CO)(10)(L)(L = 2,2'bipyridine (bpy), 2,2'-bipyrimid ine (bpym), 2,3-dipyrid-2-ylpyrazine (dpp), and 2,3-dipyrid-2-ylbenzoq uinoxaline (dpb)) were synthesized, and their low-energy Os --> L (MLC T) transitions were characterized with resonance Raman spectroscopy. C ontinuous wave excitation into the MLCT transitions does not lead to p roduct formation. However, rapid-scan TR and nano- and picosecond time -resolved absorption spectra show the formation of a transient species (for L = bpy, bpym, and dpp) which reacts back to give the parent com plex. In toluene the transient has a lifetime of less than in nr and i s assigned to an MLCT state of the cluster. In coordinating solvents a different transient species is observed with a lifetime that strongly depends on the coordinating ability of the solvent and the basic prop erties of L (200-900 ns in tetrahydrofuran (THF), 5-20 s in CH3CN). Th e transient species could even be obtained as a stable photoproduct by reaction in 2-MeTHF at 113 K and upon irradiation at room temperature in the presence of a halide salt, The transient species is proposed t o be the zwitterion Os-(CO)(4)-Os(CO)(4)-Os+(CO)(2)(S)(L). The positiv e charge of the metal in the Os+(CO)(2)(S)(L) moiety is evident from t he Raman and H-1-NMR spectral the coordination of the solvent from its large influence on the lifetime, and the absorption maximum of the tr ansient intermediate, Since the zwitterion. is already formed within 5 0 ps after the laser pulse, it is most probably produced by reaction f rom the MLCT state(s). A coordinating solvent molecule may initiate th is reaction by coordination to the metal center of the Os(CO)(2)(alpha -diimine) fragment in its MLCT state. The extra negative charge donate d by the solvent may then cause a heterolytic splitting of an Os-Os bo nd with formation of thp zwitterion.