RbCr(III)(C2O4)2.2H2O, Cs2Mg(C2O4)(2).(-)4H2O and Rb2Cu(II)(C2O4)2.2H2O: three new complex oxalate hydrates.

Rubidium chromium(III) dioxalate dihydrate [diaquabis(micro-oxalato)chromium(III)rubidium(I)], [RbCr(C2O4)2(H2O)2], (I), and dicaesium magnesium dioxalate tetrahydrate [tetraaquabis(mu-oxalato)magnesium(II)dicaesium(I)], [Cs2Mg(C2O4)2(H2O)4], (II), have layered structures which are new among double-metal oxalates. In (I), the Rb and Cr atoms lie on sites with imposed 2/m symmetry and the unique water molecule lies on a mirror plane; in (II), the Mg atom lies on a twofold axis. The two non-equivalent Cr and Mg atoms both show octahedral coordination, with a mean Cr-O distance of 1.966 A and a mean Mg-O distance of 2.066 A. Dirubidium copper(II) dioxalate dihydrate [diaquabis(mu-oxalato)copper(II)dirubidium(I)], [Rb2Cu(C2O4)2(H2O)2], (III), is also layered and is isotypic with the previously described K2- and (NH4)2Cu(II)(C2O4)(2).2H2O compounds. The two non-equivalent Cu atoms lie on inversion centres and are both (4+2)-coordinated. Hydrogen bonds are medium-strong to weak in the three compounds. The oxalate groups are slightly non-planar only in the Cs-Mg compound, (II), and are more distinctly non-planar in the K-Cu compound, (III).