Several clusters Os 3 (CO) 10 (R-diimine) (R-diimine ) pyridine-2-carbaldehyde N-R-imine or 1,4-di-R-1,4-diazabutadiene) were synthesized and studied with respect to their spectroscopic and photochemical properties. According to the resonance Raman spectra the visible absorption band of these clusters belongs to electronic transitions having Os-to-R-diimine charge transfer (MLCT) character with a variable degree of π-delocalization within the Os(R-diimine) moiety. Upon irradiation into these transitions zwitterions -Os(CO) 4 -Os(CO) 4 -Os + -(S)(CO) 2 (R-diimine) are formed in coordinating solvents (S) and biradicals • Os(CO) 4 -Os(CO) 4 -Os + (CO) 2 (Rdiimine •-) in noncoordinating solvents and in THF at ambient temperature. The zwitterions live seconds in nitrile solvents and minutes in pyridine, and they largely regenerate the parent clusters. Quantum yields of zwitterion formation are wavelength independent and range from 10 -2 to 3 × 10 -2 with an activation energy varying from 440 to 720 cm -1 . For one of the clusters the quantum yield of zwitterion formation in pyridine was studied in dependence of applied pressure. The activation volume ∆V ‡ ) +7.0 ( 0.5 cm 3 mol -1 derived from these measurements indicates that the effect of bond cleavage is partially offset by coordination of the solvent. In apolar solvents biradicals are formed instead of zwitterions, which could be detected with nanosecond timeresolved absorption spectroscopy, while their adducts with nitrosodurene were observed with EPR spectroscopy. Their lifetimes vary from 5 ns to 1 µs depending on the solvent and the R-diimine. The biradicals transform into zwitterions in the presence of a Lewis base. In addition, they produce with low efficiency an isomeric product in which the R-diimine bridges between two Os atoms. The formation of very similar photoproducts (biradicals, R-diimine-bridged isomeric products, charge-separated species) as in the case of binuclear metal-metal-bonded complexes such as (CO) 5 MnMn(CO) 3 (R-diimine) points to the occurrence of a primary photoprocess in which an Os-Os bond is broken homolytically. This reaction most likely occurs from a reactive 3 σπ* state after surface crossing from the unreactive but optically accessible MLCT states.

The photosubstitution reactions of M(CO) 4 (phen) (M ) Cr, Mo, W; phen ) 1,10-phenanthroline) with PR 3 (R ) Me, Bu n , Ph) to form M(CO) 3 (PR 3 )(phen) were studied as a function of excitation wavelength, entering nucleophile concentration, and pressure. Ligand field photolysis in general results in a dissociative substitution mechanism, whereas charge-transfer photolysis can, depending on the nature of M and PR 3 , proceed according to an associative mechanism. The chemical and physical variables studied result in a systematic tuning of the photosubstitution mechanism. Nucleophile concentration, excitation wavelength, and pressure dependencies reveal unique mechanistic information. The results are discussed in reference to available literature data, and a complete mechanistic analysis is presented.

The influence of the entering nucleophile, irradiation wavelength, and pressure on the quantum yield for the photosubstitution of M(CO) 4 phen (M ) Mo, W) to produce M(CO) 3 -(L)phen (L ) PMe 3 , PPh 3 ) was investigated in toluene at 298 K. From the pressure dependence of the quantum yield apparent volumes of activation could be determined as a function of irradiation wavelength. These could be analyzed in terms of contributions arising from dissociative ligand field excitation and associative metal-to-ligand charge transfer excitation. The influence of steric hindrance on the entering ligand (PPh 3 > PEt 3 > PMe 3 ) controls the contribution of the associative charge-transfer photosubstitution reaction. An overall mechanistic picture is presented and discussed in reference to available literature data.