Reduction of a range of amido- and aryloxy-aluminum dihydride complexes, e.g. [AlH2(NR3){N(SiMe3)2}] (NR3=NMe3 or N-methylpiperidine (NMP)), with β-diketiminato dimagnesium(I) reagents, [{(ArNacnac)Mg}2] (ArNacnac=[HC(MeCNAr)2]−, Ar=mesityl (Mes) or 2,6-xylyl (Xyl)), have afforded deep red mixed valence aluminum hydride cluster compounds, [Al6H8(NR3)2{Mg(ArNacnac)}4], which have an average Al oxidation state of +0.66, the lowest for any well-defined aluminum hydride compound. In the solid-state, the clusters are shown to have distorted octahedral Al6 cores, having zero-valent Al axial sites and mono-valent AlH2− equatorial units. Several novel by-products were isolated from the reactions that gave the clusters, including the Mg−Al bonded magnesio-aluminate complexes, [(ArNacnac)(Me3N)Mg−Al(μ-H)3[{Mg(ArNacnac)}2(μ-H)]]. Computational analyses of one aluminum hydride cluster revealed its Al6 core to be electronically delocalized and to possess one unoccupied and six occupied, skeletal molecular orbitals. © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.