Research on simple [FeFe] hydrogenase model systems of type (μ-S2R)[Fe(CO)3]2 (R = C2H4 (edt), C3H6 (pdt) which have been Shown to function as robust electrocatalysts for proton reduction, provides a reference to understand the electronic and vibrational properties of the active site of [FeFe] hydrogenases and of more sophisticated model systems. In this study, the solution arid solid state Raman spectra of (μ-edt)[Fe(CO)3]2 and of the corresponding 13CO-labeled complex are presented and analyzed in detail, with focus on the v(CO) and v(Fe-CO)/δ(Fe-CO) vibrational regions. These regions are specifically important as vibrations involving CO ligands serve as probes for the "electron richness" of low-valent transition metal centers and the geometric structures of the complexes. The obtained vibrational spectra have been completely assigned in terms of the v(CO), v(Fe-CO), and δ(Fe-CO) modes, and the force constants of the important CO and Fe-CO bonds have been determined using our Quantum Chemistry Centered Normal Coordinate Analysis (QCC-NCA). In the 400-650 cm-1 region, fifteen mixed v(Fe-CO)/O(FeCO) modes have been identified. The most prominent Raman peaks at 454, 456, and 483 cm-1 correspond to a combination of v(Fe-CO) stretching and δ(Fe-CO) linear bending modes. The less intense peaks al 416 cm-1 and 419 cm-1 correspond to pure δ(Fe-CO) linear bends. In the v(CO) region, the v(CO) normal modes at lower energy (1968 and 1964 cm-1) are almost pure equatorial (eq) v(CO) stretching vibrations, whereas the remaining four v(CO) normal modes show dominant (CO)eq (2070 and 1961 cm-1) and (CO)ax (2005 and 1979 cm ; ax = axial) contributions. Importantly, an inverse correlation between the f(CO)ax/eq and f(Fe-CO)ax/eq force constants is obtained, in agreement with the idea that the Fe(l)-CO bond in these types of complexes is dominated by π-backdonation, Compared to the reduced form of [FeFe] hydrogenase (Hred), the v(CO) vibrational frequencies of (μ-edt)[Fe(CO)3]2 are higher in energy, indicating that the dinuclear iron core in (μ-edt)[Fe(CO)3]2 is less electron rich compared to H,00 in the actual enzyme. Finally, quantum yields for the pholodecomposition of (eμ-edt)[Fe(CO)3]2 have been determined. [ABSTRACT FROM AUTHOR]