Review article Trigonal bipyramidal iron(III) and manganese(III) oxo, sulfido, and selenido complexes. An electronic-structural overview Jeanet Conradie a,b , Espen Tangen a , Abhik Ghosh a, * a Department of Chemistry, University of Tromsø, Breivika, N-9037 Tromsø, Norway b Department of Chemistry, University of the Free State, 9300 Bloemfontein, South Africa Received 10 December 2005; received in revised form 10 January 2006; accepted 10 January 2006 Available online 28 February 2006 Abstract Using density functional theory calculations, we have carried out a broad survey of trigonal bipyramidal iron(III) and manganese(III) oxo, sulfido, selenido, and hydroxo complexes, with tripodal tetradentate ‘‘triureidoamine’’ supporting ligands. The calculations repro- duce the experimentally observed high-spin states of these compounds; a multifunctional analysis suggests that the high-spin nature of these species follows largely from their trigonal bipyramidal geometry. In conjunction with earlier calculations, the present study pro- vides a broad overview of spin density profiles in iron-oxo species in general. Iron-oxo d p –p p interactions invariably result in a substantial spin density on the oxygen, which in turn may be significantly tuned by hydrogen bonding interactions. The oxygen spin densities are smaller in analogous manganese-oxo species, indicating that manganese is less adept at p-bonding than iron, which parallels earlier find- ings on porphyrin systems. The Fe III –S/Se spin density profiles provide one of the first confirmations in a transition metal context of Schleyer’s prediction that the heavier p-block elements are as effective as their second-row congeners in terms of their p-donating ability. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Iron-oxo; Trigonal bipyramidal; Manganese; Nonheme; Sulfur; Selenium 1. Introduction Terminal iron-oxo intermediates, long known only for hemes [1], have recently been detected and characterized for nonheme systems, both synthetic [2,3] and enzymatic [4,3]. While several synthetic nonheme S = 1 Fe IV O inter- mediates have been spectroscopically characterized and one even structurally characterized [5], only one nonheme enzymatic Fe IV O species – an S = 2, presumably trigonal bipyramidal (TBP) intermediate of the a-ketoacid depen- dent enzyme TauD [4,3] – has been detected so far. Earlier, we presented a broad quantum chemical survey of the elec- tronic structure and bonding of all these species [6]. How- ever, a unique group of S = 5/2 TBP Fe III E (E = O, S, Se) complexes [7], synthesized by Borovik and co-workers and somewhat reminiscent of the TBP TauD intermediate, remains inadequately explored by quantum chemical means, although a brief DFT study has been reported [8]. We present here a first systematic DFT survey of this fam- ily of compounds, as well as that of their S = 2 Mn III E analogues. Fig. 1 depicts a schematic diagram of the structure of the TBP Fe III O complex, the first complex in this family to be reported by Borovik and co-workers [9]. Also depicted in Fig. 1 are three ‘‘triureidoamine’’ ligand systems – L 1 ,L 2 , and L 3 – that we have used in our computational modeling. The L 3 ligand, which protects the Fe III E group with both steric effects and hydrogen bonding interactions, is the one actually used experimentally [7];L 2 is similar to L 3 but lacks the t-butyl groups of the latter, while the simplest ligand L 1 also lacks the hydrogen bonding interactions present in L 2 and L 3 . Throughout this study, we used spin-unrestricted DFT calculations with full symmetry- unconstrained geometry optimizations, the VWN local 0162-0134/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2006.01.029 * Corresponding author. Tel.: +47 77644072; fax: + 47 77644765. E-mail address: abhik@chem.uit.no (A. Ghosh). www.elsevier.com/locate/jinorgbio Journal of Inorganic Biochemistry 100 (2006) 707–715 JOURNAL OF Inorganic Biochemistry