The homoleptic ytterbium(II) tetraalkylaluminate complexes {Yb[AlR4]2}n have been obtained according to a silylamide elimination reaction from Yb[N(SiMe3)2]2(THF)2 and excess AlR3 (R = Me, Et, iBu). While the tetramethylaluminate derivative is a pyrophoric powder which is insoluble in aliphatic and aromatic hydrocarbons, the ethyl and isobutyl congeners are readily soluble in n-hexane. Perethylated polymeric {Yb[AlEt4]2}n is constituted formally of the two molecular fragments [Yb(AlEt4)]+ and [Yb(AlEt4)3]-, forming an intricate three-dimensional network in the solid state. Both fragments are linked by bridging α-carbon atoms and secondary Yb···H−C agostic interactions combining μ,η1, μ,η2, and μ,η3 coordination modes which result in remarkably short Yb···Al (2.809(2) Å) and a large range of Yb···C (2.649(5)−3.364(6) Å) distances. DFT calculations on the molecular fragments [Yb(AlEt4)3]- and [Yb(AlEt4)]+ reproduced the X-ray geometry remarkably well. Moreover, the theoretical investigations on model systems for the aluminate coordination support the highly fluxional nature of the aluminate coordination (ΔE(η2→η3) = −8 kcal/mol), which is also indicated by solution NMR spectroscopy. A topological analysis of the total electron density of the μ,η2-bonded aluminate ligand in the benchmark systems Y(AlR4)3 (R = Me, Et) revealed the presence of two bond critical points between the Y−Cb and Cb−Al bonds (Cb = bridging carbon atom) and thus suggests a hypervalent character of the bridging carbon atom.