The alternative mechanisms for H2 activation with 2a both involve fragmentation of the heterometallic into monomeric units. To probe this possibility, we conducted double-label cross-over experiments. Complex 2c was prepared by salt metathesis reaction of Na[Fe(η 5 -C5H4CH3)(CO)2]2 with [ZnCl{CH(CMeN-2,6-Et2C6H3)2}] and exposed to H2 to generate the corresponding dihydride species 3c. Then, a mixture of 2a and 2c were dissolved in benzene-d6 and the mixture was exposed to 1 bar of dihydrogen and the 390 nm LED lamp. After 4 hours, an equimolar ratio of the four hydride species 3a, 3c, 3d and 3e was detected by H NMR experiments (Scheme 3a). The formation of cross-over products is consistent with fragmentation of the heterometallic under the reaction conditions. However, the independent mixture of 3a and 3c, also generated the mixed hydrides 3d and 3e, under the same photochemical conditions, meaning that the fragmentation of the dihydride could also account for the products observed. To further support that fragmentation comes from the initial adducts via metal-metal bond cleavage, a solution of 2a and 2b was treated under the same reaction conditions in the absence of the H2 atmosphere (Scheme 3b). After 4 hours, the mixed iron product [Fe2(η 5 -C5H4CH3)(η 5 -C5H5)(CO)4] was detected, alongside 2d and 2e. Adduct 2e was prepared independently via the salt metathesis route and spectroscopic data match those of the cross-over experiment. These cross-over experiments suggest that breaking of the metal-metal bond can occur under photochemical conditions.