Photoproduction of dihydrogen (H-2) by a low molecular weight analogue of the active site of [NiFe] hydrogenase has been investigated by reduction of the [NiFe2] cluster, 1, by a photosensitier PS (PS = [ReCl(CO)(3)(bpy)] or [Ru(bpy)(3)] [PF6](2)). Reductive quenching of the (MLCT)-M-3 excited state of the photo-sensitizer by NEt3 or N(CH2CH2OH)(3) (TEOA) generates PS center dot-, and subsequent intermolecular electron transfer to 1 produces the reduced anionic form of 1. Time-resolved infrared spectroscopy (TRIR) has been used to probe the intermediates throughout the reduction of 1 and subsequent photocatalytic H-2 production from [HTEOA][BF4], which was monitored by gas chromatography. Two structural isomers of the reduced form of 1 (1a(center dot-) and 1b(center dot-)) were detected by Fourier transform infrared spectroscopy (FTIR) in both CH3CN and DMF (dimethylformamide), while only 1a(center dot-) was detected in CH2Cl2. Structures for these intermediates are proposed from the results of density functional theory calculations and FTIR spectroscopy. 1a(center dot-) is assigned to a similar structure to 1 with six terminal carbonyl ligands, while calculations suggest that in 1b(center dot-) two of the carbonyl groups bridge the Fe centers, consistent with the peak observed at 1714 cm(-1) in the FTIR spectrum for 1b(center dot-) in CH3CN, assigned to a nu(CO) stretching vibration. Formation of 1a(center dot-) and 1b(center dot-) and production of H-2 was studied in CH3CN, DMF, and CH2Cl2. Although the more catalytically active species (1a(center dot-) or 1b(center dot-)) could not be determined, photocatalysis was observed only in CH3CN and DMF.