Namely, bis(silylene) complexes incorporating two low-valent silicon centres have been shown to react with CO to form deoxygenated ketene motifs. Disilenide anions also react with CO to effect its coupling and deoxygenation. The conversion of CO into short chain linear alcohols and aldehydes can also be promoted by either dialkyl boranes or a combination of a trialkylborane and a suitable hydride source. In this paper, we show that a tetrametallic magnesium hydride cluster is capable of coupling and deoxygenating CO to form an acetaldehyde fragment that remains bound to the cluster as an enolate. Using DFT calculations, we propose a mechanism in which the tetrameric Mg4 entity at the core of the cluster plays a key role in bond making and bond breaking. The discovery complements known examples of CO coupling with magnesium hydride reagents. The tetrametallic magnesium hydride cluster 1 was originally reported by Harder and coworkers. Related species have recently been reported by Kretschmer and coworkers. Reaction of 1 with CO (1 atm. 60 °C) led to selective formation of 2 (Figure ). 2 incorporates a new C2 fragment derived from the coupling and deoxygenation of CO. The new C2 fragment is best formulated as an enolate of acetaldehyde, formed from a 1:2 reaction stoichiometry and combination of three hydride equivalents with two molecules of CO. The reaction is under kinetic control and a single hydride site remains unreacted in 2. 2 was characterized by a series of diagnostic resonances in the 1 H and 13 C NMR spectra. The three protons of the acetaldehyde enolate appear as distinct resonances at H = 2.86 (d, J = 13.6 Hz), 3.01 (d, J = 6.0 Hz) and 4.83 (dd, J = 13.6, 6.0 Hz) ppm with a coupling pattern diagnostic of an ABX spin-system. HSQC and HMBC correlate these environments to C NMR resonances at C = 93.1 and 148.8 ppm. The unreacted hydride of 2 is also clear from the 1 H NMR spectrum appearing as a singlet at H = 2.86 ppm. A similar C2 enolate has been suggested as an intermediate in the formation of CH2=CH2 from reaction Ln4H6 clusters with CO; the complex could not be isolated but was tentatively assigned at low temperature. The origin of CO as the carbon source for 2 was unambiguously confirmed by isotope labelling experiments.