Crystallographic new light on an old complex: NaMg[Cr(oxalato) 3]·9H 2O and structure redetermination of the isomorphous aluminum(III) compound

Abstract

The crystal structure of sodium and magnesium tris(oxalato)chromato(III) nona hydrate, NaMg[Cr(C2O4)3]·9H2O, has been determined by X-ray diffraction methods. It crystallizes in the trigonal P3c1 space group with (in the hexagonal setting) a = b = 16.9635(4), c = 12.5247(3) Å, and Z = 6 molecules per unit cell. The Cr(III) and sodium ions are in octahedral environments coordinated to three oxalate molecules and arranged in a honeycomb-like layered structure perpendicular to the crystal c-axis. There are three different octahedral [Mg(H2O)6]2+ hydrated ions that fill the honeycomb holes giving rise to electrically neutral Na[Mg(H2O)6][Cr(oxalato)3] crystal slabs. Neighboring slabs are bridged by the remaining three water molecules through H-bonds and therefore the whole crystal can be more properly described as Na[Mg(H2O)6][Cr(oxalato)3]·3H2O. Also, the structure of isomorphous NaMg[Al(C2O4)3]·9H2O compound has been re-examined and compared with the chromium(III) complex. Infrared and Raman spectra of both complexes were also recorded and briefly discussed.