The tetrametallic magnesium hydride cluster 1a was originally reported by Harder and co-workers. Related species have recently been documented from Kretschmer's group. Previously we showed that 1a reacts with two equiv. CO by a deoxygenative pathway leading to the formation of a coordinated acetaldehyde enolate. Scheme 1. Reactions of a tetrametallic magnesium hydride cluster 1a with isocyanides and CO.