REVIEWS ªNon-VSEPRº Structures and Bonding in d0 Systems Martin Kaupp* Dedicated to the memory of Marco Häser (1961 ± 1997) Under certain circumstances, metal denum or tungsten enzymes), or mate- d) p bonding. Suggestions are made complexes with a formal d0 electronic rials science (e.g. ferroelectric as to which of the factors are the configuration may exhibit structures perovskites or zirconia). Moreover, dominant ones in certain situations. that violate the traditional structure their electronic structure without for- In heteroleptic complexes, the compe- models, such as the VSEPR concept or mally nonbonding d orbitals makes tition of s and p bonding of the various simple ionic pictures. Some examples them unique starting points for a gen- ligands controls the structures in a of such behavior, such as the bent gas- eral understanding of structure, bond- complicated fashion. Some guidelines phase structures of some alkaline earth ing, and reactivity of transition metal are provided that should help to better dihalides, or the trigonal prismatic compounds. Here we attempt to pro- understand the interrelations. Bents coordination of some early transition vide a comprehensive view, both of the rule is of only very limited use in these metal chalcogenides or pnictides, have types of deviations of d0 and related types of systems, because of the para- been known for a long time. However, complexes from regular coordination mount influence of p bonding. Finally, the number of molecular examples for arrangements, and of the theoretical computed and measured structures of ªnon-VSEPRº structures has in- framework that allows their rationali- multinuclear complexes are discussed, creased dramatically during the past zation. Many computational and ex- including possible consequences for decade, in particular in the realm of perimental examples are provided, the properties of bulk solids. organometallic chemistry. At the same with an emphasis on homoleptic mon- time, various theoretical models have onuclear complexes. Then the factors Keywords: bond theory ´ d0 com- been discussed, sometimes controver- that control the structures are dis- plexes ´ density functional calculations ´ sially, to explain the observed, unusual cussed in detail. They are a) metal transition metals ´ VSEPR Model structures. Many d0 systems are impor- d orbital participation in s bonding, tant in homogeneous and heterogene- b) polarization of the outermost ous catalysis, biocatalysis (e.g. molyb- core shells, c) ligand repulsion, and 1. Introduction central metal (and also with d1 and d2 configurations) have since been found, both experimentally and computationally, The development of improved models for chemical bonding which do not conform to the simple structural models. These has always benefited from specific cases in which the existing interesting ªnon-VSEPRº d0 systems and the electronic models fail. One well-known example is the failure of the factors that control their shapes are the topic of this article. widely used valence-shell electron-pair-repulsion (VSEPR[1] ) Figure 1 shows the part of the periodic table, in which model and also of simple ionic models to predict the bent metals with a formal d0 configuration may be found. The structures of some of the molecular dihalides of the heavy configuration assignment is based on the usual oxidation state alkaline earth metals (Ca, Sr, and Ba) in the gas phase.[2±10] formalism, which assumes a heterolytic distribution of the Many more complexes with a formal d0 configuration of the bonding electrons in favor of the ligands.[11] The number of different metals for which such a configuration is possible is [*] Prof. Dr. M. Kaupp remarkable indeed, and ranges from the heavier members of Institut für Anorganische Chemie ªmainº Groups 1 and 2 through the genuine transition metals Universität Würzburg Groups 3 ± 8 to the lanthanides, and even to the later members Am Hubland, 97074 Würzburg (Germany) [12, 13] 0 Fax : (49)931-888-7135 of the actinides. Many compounds with a formal d E-mail: [email protected] configuration are of practical importance, for example, as Angew. Chem. Int. Ed. 2001, 40, 3534 ± 3565 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 1433-7851/01/4019-3535 $ 17.50+.50/0 3535 REVIEWS M. Kaupp is known about the electronic origin of the structural preferences (mainly in Section 3). We will provide many experimental examples (mainly in Sections 2, 4, and 5). Our emphasis, however, will be on the interrelation between structure and bonding. It should be noted that the interaction between experimental and quan- tum-chemical studies has been tremendously fruitful in this area of research on d0 systems. In particular, ab initio and density functional theory (DFT) computations have been shown in a considerable variety of cases to be at a stage, where rather detailed, qualitative and quantitative, structural pre- Figure 1. Range of occurrence of systems with formal d0 configuration dictions can be made. Some of the aspects of the unusual throughout the periodic table. Shaded areas indicate the range found. structures of d0 complexes have already been mentioned in Darker shading indicates increasing covalency. other contexts in previous reviews (see, e.g., refs. [10a, 16 ± 18]). d1 Complexes and low-spin d2 complexes frequently exhibit distortions very similar to those of their d0 analogues, catalysts, in important materials (see Section 5), or in bio- although the single or double occupation of one nonbonding logical systems.[14, 15] Moreover, as d0 complexes lack non- d orbital typically reduces the tendency to distort. We will bonding d electrons by definition, one may view them as the concentrate on d0 complexes but will occasionally also simplest transition metal compounds and thus as convenient mention d1 and d2 examples. starting points for a better understanding of transition metal chemistry in general. It is therefore worthwhile having a closer look at the performance of simple models in describing 2. Examples of Homoleptic Complexes with bonding and the molecular shapes of such systems. ªNon-VSEPR Structuresº Very different views have been put forward on how to explain the ªnon-VSEPR structuresº of certain d0 systems. In this Section we will give some examples, both from These range from modified ionic models through ªextended experimental and from computational studies, for homoleptic VSEPRº approaches to molecular-orbital (MO) and valence- d0 systems that depart from the expected ªVSEPR struc- bond (VB) models emphasizing the covalent bonding con- turesº. The literature available has grown tremendously tributions. Based on the results of a large number of quantum- during the past few years. We will therefore not attempt to chemical calculations during the past years by many research review all the compounds studied but rather try to cover all groups, herein we will attempt to provide a unified view of the the important types of systems. The examples will be grouped factors that control the structures of d0 systems. Initially we in the order of increasing coordination numbers. will focus mainly on the simplest mononuclear, homoleptic complexes. Recently there has been increased interest in more complicated heteroleptic, as well as in multinuclear com- 2.1. Bent Dicoordinate Complexes plexes; preliminary discussions will be given in Sections 4 and 5, respectively, of the new considerations that come into play Let us start with the well-known case of the gaseous, for such systems. Rather than giving a comprehensive choice monomeric alkaline earth dihalides. Table 1 summarizes the of examples, we will try to be systematic with respect to what current knowledge (see ref. [10] for more details) about the Martin Kaupp, born 1962 in Stuttgart, studied Chemistry in Stuttgart and Cincinnati. After his diploma thesis 1989 in Stuttgart in theoretical chemistry with Hermann Stoll, he moved to Erlangen, where he received his doctorate 1992 performing quantum-chemical studies with Paul von Rague Schleyer. Following a stay 1992/1993 at the Max-Planck-Institut für Festkörperforschung in Stuttgart with Hans-Georg von Schnering, further postdoctoral work was carried out 1993/1994 with Dennis Salahub in MontreÂal. After the return to Stuttgart, the completion of the Habilitation in theoretical chemistry followed in 1997. Since the end of 1999, he is a Professor at Universität Würzburg. Research interests cover a wide- range of theoretical and computational chemistry. The topic of the present review reflects a general interest in structure, bonding, and reactivity throughout the entire periodic table. A further central field of research is the development and application of density functional methods for the calculation and interpretation of NMR and EPR parameters, extending their applicability to compounds of heavier elements. In this context, but also in many other areas of chemistry, he is is particularly interested in the influence of relativistic effects, and in their quantum-chemical treatment. 3536 Angew. Chem. Int. Ed. 2001, 40, 3534 ± 3565 d0 Complexes REVIEWS Table 1. Preferred structures of the monomeric alkaline earth dihalides matrix-isolation infrared (IR) and density functional study [a] [MX2]. has found bent structures also for almost all lanthanide X: F Cl Br I dihydrides.[21] M: A computational comparison of a range of [MX2] com- [6] Be linear linear linear linear plexes (M Ca, Sr, Ba; X Li, BeH, CH3,NH2 , OH, F) has Mg linear linear linear linear indicated that the dimethyl complexes [M(CH3)2] also exhibit Ca quasilinear quasilinear quasilinear quasilinear an even larger tendency to bend than the corresponding Sr bent quasilinear quasilinear quasilinear difluorides, for reasons we will discuss in Section 3.8. Indeed, (DE 6.6) [22] lin Eaborn et al. have prepared a number of unsolvated [MR2] Ba bent bent bent quasilinear complexes (M Ca, Eu, Yb) with very bulky alkyl ligands (DElin 20.9) (DElin 6.8) (DElin 4.5) (R C[Si(CH3)3]3) and structurally characterized them by 1 [a] Linearization energies DElin [kJ mol ] are from Configuration inter- X-ray diffraction.