A compound widely used in the separation of the enantiomers of Δ,Λ-[Ru(bipy)3]2+ (bipy = 2,2′-bipyridine) and originally described as “a curious lattice compound” with the formula Δ-[Ru(bipy)3]3[Sb2(R,R-tart)2]2I2·18H2O (tart = tetradeprotonated, carboxyl and hydroxyl, tartaric acid anion) has been crystallographically characterized as this species with a slightly higher degree of hydration (19.5H2O). The crystal lattice has a layered structure in which sheets containing Δ-[Ru(bipy)3]2+ cations and iodide anions alternate with those containing [Sb2(R,R-tart)2]2- anions and water. The role of the iodide ions, which lie in pseudohexagonal cavities formed by the array of three inequivalent but very similar Δ-[Ru(bipy)3]2+ cations, all having their C3 axes directed very close to perpendicular to the sheet, appears to be to favor the orientation of the maximum number of CH units toward the [Sb2(R,R-tart)2]2- entities and to engender formation of (Δ-[Ru(bipy)3]2+)3([Sb2(R,R-tart)2]2-)2 groups optimizing the association of the D3-symmetric cations with the C2-symmetric anionic resolving agent. This may explain the known superiority of the resolution of Δ,Λ-[Ru(bipy)3]2+ with the anion mixture of I- and [Sb2(R,R-tart)2]2- over that with just [Sb2(R,R-tart)2]2- alone.