MICROREVIEW DOI: 10.1002/ejic.200700076 A Critical Review of Electronic Effects in Enediamido and α-Diimino Complexes of the Group 4 Metals Carlo Mealli,* [a] Andrea Ienco, [a] Andrew D. Phillips, [a] and Agustín Galindo* [b] Keywords: N ligands / Group 4 metals / Metallacycles / Density functional calculations / Electronic structure The α-diimino chelating ligand 1,4-diaza-1,3-butadiene, or its dianionic enediamido form (generically defined as DAD), forms a variety of complexes with group 4 transition metals either with or without other co-ligands. The known species are either pseudo-octahedral [L 4 M(DAD)] or tetracoordinate [L 2 M(DAD)] with a lower degree of metal saturation in the latter case. Additionally, there are a number of known CpML n species and homoleptic complexes with the formula [M(DAD) 3 ] or [M(DAD) 2 ]. The M-DAD metallacycle is gen- erally either planar or folded at the N···N vector, the latter arrangement being observed when the number of co-ligands is reduced and hence there is insufficient saturation at the metal. The conformation also depends on the relative elec- tron population of DAD, which is a noninnocent ligand and exists as a continuum between the neutral and dianionic res- onance forms. Herein some theoretical concepts, that have 1. Introduction This article analyzes the bonding capabilities of ligands based on the 1,4-diaza-1,3-butadiene or o-diiminobenzene skeleton (generally abbreviated as DAD) towards a transi- tion metal of group 4. The electron population in these molecules varies depending on the nature of the coordi- nated metal atom. [1] Thus, the chelate is considered a neu- tral α-diimino species when bound to late transition metals as occurs, for example, in the nickel(II) and palladium(II) complexes, which are capable of converting ethylene into high molecular weight polymers with low polydispersity. [2] More commonly, the ligand behaves as an enediamido di- anion with early transition metals. The corresponding com- plexes have been proposed as a more versatile alternative to metallocene catalysts for polymerization reactions. [3] With a few exceptions, the coordination of DAD in the enediamido form is characterized by a nonplanar arrange- ment of the five-membered metallacycle (an N–N folded envelope). We have already dedicated a number of theoreti- [a] ICCOM, CNR, Via Madonna del piano, 50019 Sesto Fiorentino, Firenze, Italy E-mail: mealli@iccom.cnr.it [b] Departamento de Química Inorgánica, Universidad de Sevilla, Aptdo. 553, 41701 Sevilla, Spain E-mail: galindo@us.es Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2007, 2556–2568 2556 previously been developed to describe the bonding capabili- ties of DAD toward different triads of transition metals, are extended to complexes of the group 4 metals. New electronic aspects of the [M(DAD) 3 ] complexes are described together with a completely new analysis of the [M(DAD) 2 ] complexes. These exhibit the planar/bent conformational dichotomy, which may have an electronic and/or steric origin as it is af- fected by the type of substituent at the nitrogen atoms. A reasonable justification of the observed conformational pref- erences is found from a combination of density functional and QTAIM analyses. The latter clearly demonstrates that a peculiar intramolecular hydrogen bonding can contribute to the stabilization of planar-type structures. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) cal studies to interpreting the general bonding capabilities of these ligands with metal atoms of groups 5, [4] 6, [5] and 8. [6] Herein, the dichotomy between the diimino and enedia- mido forms (see Scheme 1), which is a classic example of noninnocent behavior, [7] is discussed on the basis of the elec- tronic and structural characteristics of the ML n metal frag- ment (M = Ti, Zr, Hf). Scheme 1. Diimino vs. enediamido resonance forms in ligands de- rived from the 1,4-diaza-1,3-butadiene or o-diiminobenzene skeletons (DAD).