A Seven-Coordinate Structure of Iron(II)–Ethylenediamine-N,N,N′,N′-tetraacetato Complex as Determined by X-Ray Crystal Analysis

A single-crystal X-ray analysis has revealed that the FeII–edta complex, formulated as Na2[Fe(edta)(H2O)]·2NaClO4·6H2O, has a seven-coordinate structure close to a mono-capped trigonal prism (C2v-CTP). The crystal data are as follows: Orthorhombic, Pmnb, a = 11.404(1), b = 23.573(2), c = 9.915(1) Å, and Z = 4. In the complex anion, edta serves as a hexadentate ligand with its two N atoms at a unique edge and four O atoms at the corners of a quadrilateral face, which is capped with a water molecule. The presence of the labile water molecule in the first coordination sphere accounts for the kinetic lability of the FeII–edta complex, which has been frequently pointed out. A qualitative comparison of the molecular structures was made between the seven-coordinate FeII– and FeIII–edta complex anions. The steric and electronic requirements for seven-coordination are also discussed concerning the transition-metal edta complexes.