The common role of water molecule and lone electron pair as a bond-valence mediator in oxalate complexes: the crystal structures of Rb2(C2O4) · H2O and Tl2(C2O4)

Abstract

The crystal structure of rubidium oxalate monohydrate, Rb2(C2O4) · H2O, has been refined using an imaging-plate diffractometer system and graphite-monochromatized MoK α radiation. The complex crystallizes in the monoclinic system, space group C2/c, with unit cell dimensions of a = 9.617(6), b = 6.353(5), c = 11.010(8) Å, β = 109.46(3)°, V = 634.2(8) Å3, and Z = 4. The structure was solved and refined to R = 0.026 for 2646 independent reflections [I o > 2σ(I o)]. In Rb2(C2O4) · H2O, oxalate anions [(C2O4)2–] and cations (Rb+) constitute a two-dimensional layered structure parallel to (001); interlayered water molecules (H2O)0 occupy a well-defined position and form hydrogen bonds to the layers to prop up the structure, where (H2O)0 is structurally an essential component. The relationship among the crystal structures of M2(C2O4) · nH2O (M = Li+, Na+, K+, Rb+, Cs+, Tl+, NH4 +, Ag+; n = 0, 1, 2) showed that ionic radii of the cations decide whether the corresponding oxalate complexes have water molecules or not. On the other hand, anhydrous thallium oxalate, Tl2(C2O4), has a similar structure to Rb2(C2O4) · H2O except for the water molecules. Tl+ cation exhibits stereochemical activity of its 6s2 lone electron pairs (LEP), which is absent from Rb+ in the latter oxalate. Comparison between interlayer spaces occupied by LEP and (H2O)0 in these two structures reveals the existence of the first uncharged substitution: 2 × LEP → (H2O)0, where the directions of the former occupants are opposite to each other, and nearly perpendicular to a dipole direction of the latter. In the layer sites of these two structures, two LEP of Tl+ seem to work like a dipole moment of the water molecule in Rb-oxalate monohydrate by shifting away from the nucleus to stabilize the structure of Tl2C2O4. The crucial role common to water molecules and LEP which acts as a bond-valence mediator in oxalate complexes is discussed quantitatively in terms of a valence-matching principle.