Structural characterization and conformational analysis of (aqua)(ethylenediamine-N , N , N 0 -triacetato)chromium(III) monohydrate complex. Crystal structure of the cis-equatorial isomer of [Cr(ed3a)(H 2 O)]  H 2 O Sonja Grubi  si  c a , Maja Gruden-Pavlovi  c a , Svetozar R. Niketi  c a, * ,1 , Sumio Kaizaki b , Narumi Sakagami-Yoshida b a Department of Chemistry, Faculty of Science, University of Belgrade, P.O. Box 158, YU-11001 Belgrade, Yugoslavia b Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan Received 19 April 2003; accepted 26 June 2003 Published online: 22 July 2003 Abstract The structure of the cis-equatorial isomer of [Cr(ed3a)(H 2 O)]  H 2 O (ed3a ¼ ethylenediamine-N , N , N 0 -triacetate ion) was deter- mined by X-ray diffraction method. The complex crystallizes in the monoclinic space group P2 1 /n, a ¼ 7:004ð1Þ  A, b ¼ 15:958ð2Þ  A, c ¼ 11:046ð1Þ  A, b ¼ 97:16ð1Þ°, and Z ¼ 4. Conformational analysis of the three possible geometrical isomers, trans(H 2 O,N 0 ), trans(H 2 O,N H ), and trans(H 2 O,O) of [Cr(ed3a)(H 2 O)] moiety, performed using the consistent force field (CFF) program with the recently developed parameters for EDTA-type complexes, yielded structural details and energies of the minimized form for each of the isomers. Calculated energies showed that the cis-eq isomer is the most stable one, with the geometry in a very good agreement with the crystallographic structure. Comparison of the molecular mechanics calculations with those for the analogous [Cr(ed3p)(H 2 O)]  H 2 O (ed3p ¼ ethylenediamine-N , N , N 0 -tripropionate ion) revealed some general patterns for the conformational preference of EDTA-type complexes. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Cr(III) complexes; Multidentate chelates; X-ray crystal structure; Molecular mechanics (MM) 1. Introduction Sexidentate edta complexes are susceptible to sub- stitution reactions in which certain unidentate ligands can displace an in-plane (G) glycinato ring. In general, when edta (edta ¼ ethylenediaminetetraacetate ion) and ed3a (ed3a ¼ ethylenediamine-N ; N ; N 0 -triacetate ion) are coordinated as quinquedentate ligands three geo- metrical isomers (commonly labeled as cis-eq, cis-polar, and trans-eq) are possible for the octahedral structure (Scheme 1). Recently, trans-eq-[Cr(ed3p)(H 2 O)] (ed3p ¼ ethylenediamine-N , N , N 0 -tripropionate ion) complex containing three six-membered b-alaninato-type rings has been a subject of the conformational study [1], with the aim to elucidate anomalous C–N bond cleavage [2– 4] and unprecedented C–N bond recombination [5]. Molecular mechanics showed that the trans-eq isomer is the most stable one for the [Cr(ed3p)(H 2 O)] and this result was in agreement with the reported X-ray struc- tures [1,6]. The structural changes in quinquedentate ed3a-type ligand are expected to have influence on the geometry of the octahedral species. In order to investigate the in- ternal strain in these complexes and to elucidate factors which determine the preferred conformation and the geometry of [Cr(ed3a)(H 2 O)] structures, we performed molecular mechanics (MM) calculations for each of the geometrical isomers, using previously developed force Inorganic Chemistry Communications 6 (2003) 1180–1184 www.elsevier.com/locate/inoche * Corresponding author. E-mail address: nik@chem.bg.ac.yu (S.R. Niketi c). 1 Roche Research Fellow, 2001–2002. 1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1387-7003(03)00220-X