We have previously reported the synthesis, properties and reactivity of the heterometallic complex 1. This species possesses a distorted structure with an aluminium-centred LUMO and a HOMO that is largely Fe centred and is responsible for the Fe-Al interaction. Reactions typically occur through initial association a substrate (e.g. pyridine, acetonitrile, alkyne, alkene) at the electrophilic aluminium centre, followed by pathways in which the Fe centre acts as either a base or a nucleophile. Addition of CO (1 bar) to a solution of 1 in C6D6 at 25 °C resulted in an immediate colour change from deep red to pale red-orange. Analysis of the reaction mixture by NMR spectroscopy revealed the complete consumption of 1 and the formation of a new species 2a in >95 % yield (Scheme 1). In its 31 P{ 1 H} NMR spectrum, 2a features two broad resonances at δP = 18.60 and 9.42 ppm, integrating to 1P and 2P each, indicating two different coordination environments of the three P ligands. In the 1 H NMR spectrum, 2a shows two distinct hydride environments integrating to 1H each. These are assigned as a bridging Fe-μ-H-Al at δH = -15.00 ppm (q, JP-H = 25.0 Hz), and terminal Al-H observed as a broad resonance at δH = +5.72 ppm. EXSY experiments detected a lack of chemical exchange between these environments at room temperature. The IR spectrum of 2a features a characteristic strong vibration at υ = 1823 cm -1 assigned as the C=O stretch, typical for a semi-bridging carbonyl ligand (vide infra). The Al-H stretching mode was also identified at υ = 1674 cm -1 , significantly redshifted from the bridging hydrides in 1 (υ = 1795 and 1716 cm -1 ). 2a could be isolated by crystallisation in a 79 % yield and was found to be stable at room temperature under an inert atmosphere over multiple months in its pure form. Characterisation by X-ray crystallography confirmed the presence of an iron-bound CO ligand (Figure ). Both the bridging and the terminal hydride ligands were located in the Fourier difference map and the solid-state structure is consistent with the solution NMR data. The binding of the carbonyl ligand appears to be non-reversible as the CO cannot be removed under vacuum. Complexes 2a and 2d feature significantly longer Fe-Al distances compared to complex 1 (2a: 2.3948(19) Å, fsr = 0.99; 2d: 2.4314(10) Å, fsr = 1.01, d(Fe-Al); 1: 2.2176(13) Å, fsr = 0.92). As a measure of the bridging nature of the CO/isocyanide ligands in 2a and 2d we employed the metrics introduced by Curtis and Mankad. These show that 2a is best described as a semi-bridging carbonyl complex (α = 0.48), whereas 2d is a borderline case for semi-bridging character (α = 0.61, see SI for details). Given the known reactivity of 1 along with the formal 18-valence electron count, it seems unlikely that strong σ-donors such as CO and CNR can bind to the Fe metal centre by an associative mechanism. A deeper analysis of the bonding in 2a-d and their mechanisms of formation suggests this process occurs indirectly through initial through coordination at Al followed by a yet unknown (or unusual) form of dyotropic rearrangement (vide infra). The electronic structure of 2a was further investigated through DFT calculations (Figure ). These calculations suggest the CO ligand resides on Fe with some semi-bridging character and that one of the hydride ligands has migrated to a terminal position on Al. Wiberg Bond Indicies (WBIs) from Natural Bond Orbital (NBO) calculations suggest that the Fe-CO (0.84) bond has higher covalency than the Al-CO (0.30) bond, and that of the two hydrides one is primarily associated with Fe (Fe-H 1 0.40, Al-H 1 0.21) and the other is terminally bound to Al (Fe---H 2 0.01, Al-H 2 0.70). The Fe-Al bond exhibits somewhat reduced covalency in comparison to 1 as corroborated by NBO and QTAIM analysis (Fe-Al WBIs 1: 0.50; 2a: 0.26, ρ(r) at the Fe-Al BCP: 1: 0.062; 2a: 0.055). Quantum Treatment of Atoms in Molecules (QTAIM) calculations support this assignment returning bond paths between Fe and Al, Fe and C, Fe and H 1 , and Al and H 2 but not other pairs of these atoms. Analysis of the Intrinsic Bonding Orbitals (IBOs) of 2a reveals that in addition to the Fe-C σ bond, the Fe-Al bond is also partially delocalised over the C atom, supporting the semi-bridging nature of this ligand.