DFT calculations were also undertaken to study the underlying mechanism for formation of 2a-d from 1 (Figure ). In the case of 2a, the first step of the reaction is the slightly endergonic coordination of the CO ligand to the Al site of 1 (INT-1a, ΔG° = 2.0 kcal mol -1 ), followed by its migration (TS-1a, ΔG ‡ = 2.3 kcal mol -1 ) to form a bridged species (INT-2a, ΔG° = -7.6 kcal mol -1 ). This is then followed by the simultaneous but asynchronous dyotropic migration of the CO and one of the hydride ligands to form the experimentally observed product 2a (TS-2a, ΔG ‡ = -2.6 kcal mol -1 ). Overall, this transformation is highly exergonic (ΔG° = -27.4 kcal mol -1 , ΔH° = -39.8 kcal mol -1 ) and proceeds via a very low barrier (ΔG ‡ = +5.0 kcal mol -1 , ΔH ‡ = +5.0 kcal mol -1 ) agreeing with the experimentally observed nonreversibility and fast rate of the reaction. An alternative pathway involving a 1,2-migration of a hydride ligand from Fe to Al (e.g. dissociative with respect to Fe) followed by trapping with CO was also calculated, however, was found not to be competitive (ΔΔG ‡ = 13.0 kcal mol -1 , Figure ). The formation of 2b-d was calculated to proceed via analogous mechanism to that presented in Figure but with overall higher barriers (ΔG ‡ = +5.7-14.7 kcal mol -1 ), likely due to increased steric clash with the ligands (see Figures for full computed pathways). To gain further insight into the electronic changes during the rearrangement we conducted analysis of key localised molecular orbitals (LMOs) along the potential energy surface (Figure ). The LMOs were calculated using the IBO procedure and the procedure described by Vidossich and Lledóss was used to generate centroids of these LMOs (CLMOs). CLMO analysis for TS-1a revealed a shift of the former Fe-Al bond in 1 and INT-1a forming the Fe-C σ bond in INT-2a. As hinted to by lack of metal-metal bonding by NBO and QTAIM analysis earlier, no Fe-Al bonding IBO could be identified in INT-2a. Instead, the presence of distinct Fe-C and Al-C IBOs suggest that the bonding in the bimetallic core is entirely based on the bridging CO and the hydride ligands. CLMO analysis also supports the asynchronicity of TS-2a. The pre-TS part primarily consists of the transfer of the hydride from Fe to Al, and the weakening of the Al-C bond. This is followed by the strengthening of the Fe-C bond and the reestablishment of the Fe-Al bond in the post-TS stage. To preserve orbital symmetry the same IBO is assigned to the Fe-Al bond in 2a and 1, meaning that the two Fe-based bonding orbitals cross in the post-TS stage of TS-2a. The mechanism presented above is different from type I dyotropic rearrangements in organic systems (Scheme 3). In type I dyotropic rearrangements, the static carbon framework is stabilised during the transition state due to partial π-bonding enabled by the almost planar geometry of the ethylene scaffold, and thus the reaction barriers arise primarily from the weakening of the bonds between the carbon framework and migrating groups. In the current case, as the reaction coordinate is traversed, the metal-metal bond is disrupted and then re-formed. Meaning the bonding in static framework is actually weakened during the rearrangement. In addition, the majority of known type I dyotropic rearrangements are concerted. The case reported herein takes place via a stepwise mechanism through an unstable intermediate with a CO ligand occupying a bridging position followed by a dyotropic transition state. Despite these differences, the suggested mechanism is consistent with those proposed for other bimetallic species. This includes the Fe-Pt system reported by Braunstein, Knorr and Hoffmann, and first row transition metal carbonyl dimers. In the former case, variation of the R groups on the migrating SiR3 and the bridging PR2 ligands allowed for the isolation intermediates on the reaction pathway. A detailed computational study later established that the dyotropic rearrangement takes place via an intermediate where the CO ligand occupies a bridging position. This is followed by a dyotropic transition state where both the migrating silyl and CO groups occupy bridging positions across the two metal centres. For transition metal carbonyl dimers (eg.