Structural and electronic features of Group 8 metal complexes containing one a-diiminobenzene chelate ligand Adela Anillo a , Santiago Garcia-Granda b , Ricardo Obeso-Rosete a, *, Agustı ´n Galindo c , Andrea Ienco d , Carlo Mealli d, * a Departamento de Quı ´mica Orga´nica e Inorga´nica, Instituto Universitario de Quı ´mica Organometa´lica ‘Enrique Moles’, Julia´n Claverı ´a s/n, 33071 Oviedo, Spain b Departamento de Quı ´mica Fı ´sica y Analı ´tica, Facultad de Quı ´mica, Universidad de Oviedo, Julia´n Claverı ´a s/n, 33071 Oviedo, Spain c Departamento de Quı ´mica Inorga´nica, Universidad de Sevilla, Aptdo 553, 41071 Seville, Spain d ICCOM, CNR, Via Nardi 39, 50132 Firenze, Italy Received 6 August 2002; accepted 4 October 2002 Article submitted in honour of Professor Pierre Braunstein Abstract Known complexes of general formula L n M(diim ), where diim /C 6 H 4 (NH) 2 -o , a-diiminobenzene, n /3, 4 and M is a metal of Group 8 (Ru(II), in particular) are analyzed in terms of their electronic structure and associated energetics. The newly synthesized compound [(CO) 2 (PPh 3 )BrRu(C 6 H 4 (NH) 2 -o )]Br, 1, is a new member of the category. This is confirmed by the X-ray structure of 1 that is presented together with electrochemical measurements and other spectroscopic characterization. At variance with the isoelectronic L 4 M(o -phenylenediamido) compounds (M/Group 6 metal) that are affected by structural deformations, 1 and similar complexes have a regular pseudo-octahedral structure. Density functional theory calculations and qualitative MO arguments provide an explanation for the different electronic trends as well as for the diimino/diamido dichotomy of the non-innocent chelate ligand. The latter is affected, not only by the nature of the metal (energy of d orbitals), but also by the surrounding environment (three or four coligands). Moreover, the calculations allow to outline the stability trends for the loss of one coligand from L 4 M(diim ) systems when the Ru(II) complexes are chemically or electrochemically reduced. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Diiminobenzene complexes; Group 8 metals; Density functional theory calculations; Theoretical analysis 1. Introduction Transition metal complexes that contain at least one a-diiminobenzene chelate (C 6 H 4 (NH) 2 -o , diim ) are still the subject of many experimental and theoretical studies because of their effectiveness in homogenous catalysis [1]. Recently, their potentialities as DNA-binding inter- calators have been tested [2]. This ligand may adopt a variety of coordination modes in mononuclear and polynuclear complexes and, in principle, it can be the donor of a different number of electron pairs (up to five in some binuclear species [3]). Interesting properties are due to the non-innocence of diim that may be stabilized in either the uncharged diimino or the dianionic diamido form (Scheme 1). Scheme 1. * Corresponding authors. Tel.: /39-55-234 6653; fax: /39-55-247 8366. E-mail addresses: ror@sauron.quimica.uniovi.es (R. Obeso-Rosete), mealli@fi.cnr.it (C. Mealli). Inorganica Chimica Acta 350 (2003) 557 /567 www.elsevier.com/locate/ica 0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01557-8