Abstract
Addition of CO to a tetrametallic magnesium hydride cluster results in both carbon-carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C(2)OH(3)](-) which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C(2)OH(3)](-) fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C(2)O(2)H(3)](3-) fragment, which undergoes subsequent deoxygenation.