Synthesis and X-ray diffraction study of hexaamminecobalt(III) sodium ethylenediaminetetraacetate 3.5-hydrate

The title compound [Co(NH₃)₆] [Na(EDTA-4H)],3.5H₂O has been synthesized, and its crystal structure has been determined from 2064 independent single-crystal reflections measured on a diffractometer. The crystals are tetragonal, space groupP42₁2, with unit-cell parametersa = 14.618(5) andc = 10.658(4) Å. There are four formula entities in the unit cell givingD_c = 1.562(2) g cm^–3 compared withD_m = 1.56(1) g cm^–3. The structure was solved by conventional Patterson and Fourier techniques, and refined by full-matrix least-squares methods to a final agreement factor of 3.6%. The compound contains two crystallographically independent octahedral [Co(NH₃)₆]³⁺ ions, with an average Co—N distance of 1.966(6) Å, and a seven-coordinate sodium ion which is coordinated to four carboxyl oxygens, two amine nitrogens, and a water molecule which bridges to another sodium ion. The EDTA is therefore acting in its normal hexadentate manner. The range of Na—O distances is 2.25 to 2.93 Å whereas the Na—N distance is 2.46 Å. The crystal structure is additionally stabilized by extensive hydrogen bonding among coordinated ammonias, carboxyl oxygens, and water molecules.