No reaction occurred under thermal conditions. While this experiment cannot rule out the direct formation of 3a from 1a and [Fe(η 5 -C5H5)(CO)2]2 it suggests that 2a is capable of activating H2 under photolysis. The reaction was monitored by UV-vis spectra were recorded for 2a and the dihydride product 3a. Both species present similar absorption bands with a maximum absorption peak around 350 nm. The light absorption of both starting material and product at the same wavelength might explain the challenge of achieving higher yields and the instability of 3a during photolysis. From all the conditions tested, the best results obtained were: 1 bar H2, benzene solvent, 4 hours irradiation using a 390 LED lamp (3a, 45% isolated yield or 55% NMR yield). Experiments using D2 showed 92% deuterium incorporation in 3a, demonstrating that the hydride groups come from dihydrogen, rather than from other possible sources i.e. reaction with the solvent.