η6-Arene(iodidoaluminato)lanthanoid(II) complexes, {[Ln(η6-C6H5Me)(AlI4)2]}n [Ln = Sm, 1, Eu, 2, Yb, 3; C6H5Me = toluene] have been prepared by reactions of in situ generated aluminium triiodide with the corresponding lanthanoid metals and 1,2-diiodoethane in toluene (molar ratio: 2:1:1). Compounds 1–3 are polymeric and the lanthanoid(II) atom of 1 and 2 is coordinated by an η6–arene and three chelating κ (I, I′) tetraiodidoaluminate ligands, two of which are also bridging (LnI2AlI2Ln) and one is terminal. 3 is coordinated by an η6–arene, one terminal chelating κ (I, I′) and one chelating and bridging (YbI2AlI2Yb) tetraiodidoaluminate ligand, and one monodentate tetraiodidoaluminate ligand, which is also bridging (YbIAl(I)I2Yb). These are the first X-ray crystal structures of arene(tetrakisiodidoaluminato)lanthanoid(II) complexes. The first reported alkaline earth(II) (η6–arene)iodidoaluminate complexes, namely [Ca(η6-C6H5Me)(AlI4)2]n (4), {[Sr(η6-C6H5Me)(AlI4)2]n·nPhMe} (5), [Ba(η6-C6H5Me)2(AlI4)2] (6), [Ca(η6-C6H3Me3-1,3,5)(AlI4)2]n (7) and {[Sr(η6-C6H3Me3-1,3,5)(AlI4)2]n·0.5n(PhMe3)} (8) have also been obtained in an arene (toluene or mesitylene) solution by a similar method. Complex 4 is isocoordinate with 3, and 5 is isomorphous with 2. Complex 7 has eight coordinate Ca similar to 4, and complex 8 has nine coordinate Sr as in 5. Mononuclear ten coordinate complex 6 is the first example of a monomeric η6–arene sandwich complex of barium. The EuII bromidoaluminate complex {[Eu(η6-C6H5Me)(AlBr4)2]n·0.5nPhMe} (9) has been prepared from Eu metal, AlBr3, and BrCH2CH2Br in toluene and has a unit cell similar to 8 despite the different metal and arene.