The ligand field stabilization effect on the metal-ligand bond distances in octahedral metal complexes with edta-type ligands. Redetermination of the molecular structure of (ethylenediaminetriacetatoacetic acid)-(aqua)iron(III), [Fe(Hedta)(H 2O)

Abstract

The molecular structure of the title complex [Fe(Hedta)(H 2O)] (Hedta=mono-protonated ethylene- diamine- N,N,N′,N′-tetraacetate) has been redetermined by single crystal X-ray diffraction methods. The crystal data are as follows: monoclinic, space group P2 1/ c, a=8.031(2), b=8.897(2), c=17.627(4) Å, β=99.55(2)°, Z=4 and R=0.041. The complex has an octahedral structure in which the protonated edta serves as a pentadentate ligand and a water molecule completes the six coordination, the ML bond distances and LML′ bond angles redetermined are both comparable to those reported earlier, but the equatorial MO bonds trans to the MN bonds are shorter on average than the axial MO bonds, at variance with the results of the earlier structure analysis in which the R value converged to 0.12. Detailed comparison of the ML bond distances among several octahedral metal complexes with edta-type ligands including Hedta, demonstrates that the equatorial MO bonds are longer on average than the axial odes for the Cr(III), Co(III) and Rh(III) complexes to which the ligand field stabilization (LFS) contributes greatly, while the reverse holds not only for the Fe(III) complexes but also for [Al(edta)] −, [Zn(edta)] 2−, [Ga(Hedta)(H 2O)] and [Ge(Hedta)(OH)] to which there is no appreciable LFS contribution. The variation of the ML bond distances with the M ion in these complexes has been discussed in relation to the LFS of the M ion.