Structure and Bonding in Organometallic Anions of Heavy Group 14 and 15 Elements

Alkali metal organometallic complexes (containing C–metal bonds) and the frequently structrually related alkali metal amides and alkoxides have been investigated extensively both in the solid state and in solution in the past two decades. However, until recently, the related complexes containing the heavier metallic and semi-metallic p block elements and the alkali and alkaline earth metals had rarely been studied in their own right. Recent solid-state structural studies have illustrated the immense structural diversity and bonding modes to be found within these species. One of the principal focuses of recent studies has been complexes containing organometallic anions of p block metals (e.g., triorganostannates, containing R₃Sn^?) in which metal–metal bonds occur between the heavy p block metal and the alkali or alkaline earth metal and the investigation of the nature of this bonding. The development of new synthetic routes has also allowed the preparation of a variety of anionic ligands with p block metal centers which promise new opportunities in coordination chemistry. In addition, the synthesis of a family of homologous anionic π complexes has given a fresh direction in the chemistry of p block metal metallocene complexes.     

Saturated Heavier Group 14 Compounds as ‐Electron‐Acceptor (Z‐Type) Ligands

This review article describes the chemistry of transition‐metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ‐electron‐acceptor (Z‐type) ligands and discusses the characteristics of bonds between the transition metal and Z‐type ligand. Moreover, we review the iridium hydride mediated cleavage of E–X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.     

Mercury,a Structural Building Block and Source of Localized Reactivity in Extended Metal–Metal Bonded Systems

Transition metal–mercury complexes were among the first compounds of study for the concept of direct metal–metal bonding which was established more than three decades ago. Since then, a large number of such systems have been synthesized and studied. The fact that mercury is readily attached to a large variety of main group or transition metals has stimulated its use as a general building block in the systematic synthesis of mixed-metal clusters. The past decade has witnessed a rapid expansion of bimetallic cluster chemistry in which species containing mercury have played a prominent role, and which has led to the discovery of many unprecedented cluster structures and reactions. In particular, the ability of mercury to form multicenter metal–metal bonds with polynuclear cluster fragments has substantially extended its coordination chemistry which was thus far dominated by simple linear structural arrangements. Although certain structural motifs are found to be common to many of the transition metal–mercury clusters investigated to date and thus enable a relatively systematic synthetic approach, the multitude of surprising discoveries has kept the interest in the chemistry of the element itself alive. The recent discovery of the redox and photochemical reactivity of some of these systems has opened up an exciting and promising area of cluster research. Its significance for the synthetic methodology lies in the fact that the increasing redox activity of molecular carbonyl clusters on going to higher nuclearities appears to set a limit on the size of metal frameworks attainable by the standard preparative methods. On the other hand, their potential use as photochromes or redox mediaters in coupled electron-transfer reactions provides an additional stimulus for future studies in this field.     

Large Isotope Effects in Organometallic Chemistry

The kinetic isotope effect (KIE) is key to understanding reaction mechanisms in many areas of chemistry and chemical biology, including organometallic chemistry. This ratio of rate constants, k_H/k_D, typically falls between 1–7. However, KIEs up to 105 have been reported, and can even be so large that reactivity with deuterium is unobserved. We collect here examples of large KIEs across organometallic chemistry, in catalytic and stoichiometric reactions, along with their mechanistic interpretations. Large KIEs occur in proton transfer reactions such as protonation of organometallic complexes and clusters, protonolysis of metal–carbon bonds, and dihydrogen reactivity. C−H activation reactions with large KIEs occur with late and early transition metals, photogenerated intermediates, and abstraction by metal-oxo complexes. We categorize the mechanistic interpretations of large KIEs into the following three types: (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step. This comprehensive collection of large KIEs in organometallics provides context for future mechanistic interpretation.     

Condensed Metal Clusters

The chemistry of metals in low valence states is marked by the frequent occurrence of metal clusters, which are easily recognizable when they occur as molecular units. Many metal-rich compounds of transition metals with p-elements (3rd to the 6th main groups) are closely related to the corresponding halides, since they are built up from metal clusters of the same type. The clusters are however, linked together (condensed) by metal-metal bonds. This principle of construction holds particularly well in the case of the novel reduced halides of the lanthanoids.     

Visible‐Light‐Induced Nickel‐Catalyzed Negishi Cross‐Couplings by Exogenous‐Photosensitizer‐Free Photocatalysis

The merging of photoredox and transition‐metal catalysis has become one of the most attractive approaches for carbon–carbon bond formation. Such reactions require the use of two organo‐transition‐metal species, one of which acts as a photosensitizer and the other one as a cross‐coupling catalyst. We report herein an exogenous‐photosensitizer‐free photocatalytic process for the formation of carbon–carbon bonds by direct acceleration of the well‐known nickel‐catalyzed Negishi cross‐coupling that is based on the use of two naturally abundant metals. This finding will open new avenues in cross‐coupling chemistry that involve the direct visible‐light absorption of organometallic catalytic complexes.     

Platinum Group Organometallics Based on “Pincer” Complexes: Sensors,Switches, and Catalysts

Since the first reports in the late 1970s on transition metal complexes containing pincer‐type ligands—named after the particular coordination mode of these ligands—these systems have attracted increasing interest owing to the unusual properties of the metal centers imparted by the pincer ligand. Typically, such a ligand comprises an anionic aryl ring which is ortho,ortho‐disubstituted with heteroatom substituents, for example, CH₂NR₂, CH₂PR₂ or CH₂SR, which generally coordinate to the metal center, and therefore support the M−C σ bond. This commonly results in a terdentate and meridional coordination mode consisting of two metallacycles which share the M−C bond. Detailed studies of the formation and the properties of a large variety of pincers containing platinum group metal complexes have provided direct access to both a fundamental understanding of a variety of reactions in organometallic chemistry and to a range of new applications of these complexes. The discovery of alkane dehydrogenation catalysts, the mechanistic elucidation of fundamental transformations (for example, C−C bond activation), the construction of the first metallodendrimers for sustainable homogeneous catalysis, and the engineering of crystalline switches for materials processing represent only a few of the many highlights which have emanated from these numerous investigations. This review discusses the synthetic methodologies that are currently available for the preparation of platinum group metal complexes containing pincer ligands and especially emphasizes different applications that have been realized in materials science such as the development and engineering of sensors, switches, and catalysts.     

Metal-Boron Compounds – Problems and Perspectives

“True” metal-boron compounds have been known for about ten years. The bonding between the metal and boron atoms can vary widely in nature: Not only simple covalent bonds are encountered but also coordinate bonds and boron-metal multicenter bonds. Furthermore, π complexes of transition metals with boron-nitrogen systems and metal complexes containing boron(I) compounds as ligands have recently also been prepared.     

RP-Bridged Metal Carbonyl Clusters: Synthesis,Properties, and Reactions

Metal carbonyl clusters possess a complicated chemistry that is only beginning to be understood. One of the main current goals in this area is thus an understanding of their reactivity. This article describes the syntheses and reactions of clusters that contain metal carbonyl fragments bridged by a main-group element. But what is the sense of making such clusters still more complicated by the incorporation of main-group elements? The example of μ₃-bridged carbonyl clusters will serve to show that the main-group element plays an important role in the study of reaction paths; it holds the metal carbonyl fragments together even when the bonds between them are broken in the course of a reaction. Trinuclear μ₃-bridged clusters prove to be small enough to allow the analysis of typical cluster reactions (such as the reversible breaking of metal-metal bonds) in terms of single reaction steps. They are also large enough to provide surprises by their multifaceted reactivity. It will be shown that a detailed study of trinuclear RX-bridged metal carbonyl clusters (X ? N, P, As, Sb, Bi)—a very small part of carbonyl cluster chemistry—can lead to a better understanding of the general reaction principles involved.     

Low‐Valent Organometallics—Synthesis,Reactivity, and Potential Applications

General concepts for the synthesis and stabilization of low‐valent organometallic complexes of Groups 2, 12, 13, and 15 metals and common structural motifs are described. While kinetically stabilized complexes are in the focus for more than two decades, the principle of base‐stabilization only recently allowed the synthesis of unforeseen compounds. As‐prepared complexes not only show fascinating structural diversities, but exhibit also very interesting chemical properties. Low‐valent complexes are of particular interest in the synthesis of novel molecular complexes, but may also find applications as tailor‐made precursors for the synthesis of nanosized materials.     

Phosphorus Ylides in the Coordination Sphere of Transition Metals: An Inventory

Phosphorus ylides are not only classical reagents in organic chemistry, but also play an increasingly important role as novel components in organometallic compounds. These metallic “ylide complexes” are either synthesized from “preformed ylides” and coordination compounds by addition or substitution, on the building block principle, or they are formed, in sometimes complicated reactions, from phosphanes, metal complexes, and C₁ substrates in the coordination sphere of the metals. The resulting metal-carbon bonds are greatly modified in their properties by the immediate presence of the phosphonium center and often belong to the most stable of M-C structural units. The metal can come from any group of the periodic table, including the lanthanoids and actinoids. Numerous preparative and structural studies are gradually enabling us to gain an overall picture of the scope of this area of research.     

Sulfur Atoms as Ligands in Metal Complexes

The types of sulfur bonding—as sulfane or sulfide—encountered in the molecules of maingroup elements are almost unknown in the chemistry of metal complexes, where the sulfur atoms function instead as two-electron donors by bridging two metal atoms, as four-electron donors by bridging three or four metal atoms, or as six-electron donors by incorporation between four metal atoms. In such complexes, the metal-metal bond can be modified over a wide range by chemical or electrochemical variation of the number of electrons present. The readiness with which polynuclear complexes containing metals and sulfur undergo redox reactions is also utilized by Nature in the active sites of some redox proteins.     

Mixed Ligands from Group 15/16 Elements: Interface of Solid-State and Molecular Chemistry

A wide range of chemical compounds is spanned by heteroatomic ligands from Group 15/16 elements, which in Nature extend from AsS (in realgar) to [AsS₂⁻]_∞ (in sulfosalt minerals). The stabilization of labile molecules or those that do not exist in the free state by incorporation into transition metal complexes like [Cp^*₂Fe₂(AsSe)₂] ( 1 ) and the oligomerization of molecular units under the influence of metal ions or complexes to form solids or hybrid clusters with inorganic cores and peripheral organometallic ligands are the main subjects of this article. Cp*=C₅Me₅.     

Organometallic Compounds with Homonuclear Bonds between Bismuth Atoms, 70 Years after Paneth' Report on the Violet Dimethyl Bismuth Compound

The chemistry of organometallic compounds of the types (R₂Bi)₂, (RBi)_n (n = 2—5) and R₆Bi₈ is the objective of this report. Common features and differences between these bismuthanes and analogous compounds of lighter group 15 elements are discussed.     

Alkali Metal-Transition Metal π-Complexes

In this progress report a new principle is described for the synthesis of multimetal π-complexes with hitherto unknown structural features. The nickel(0)-olefin complexes 1,5,9-cyclododecatrienenickel(0) and bis(1,5-cyclooctadiene)nickel(0) react unexpectedly with main-group metals, in particular alkali metals, their hydrides, and organometallic compounds, to give nickelligand species with a surplus charge. These species endeavor to transfer the excess charge onto π-ligands such as olefins or dinitrogen. Multimetal complexes with electron rich transition metal π-ligand units can, in addition, be prepared from metallocenes, alkali metals, and unsaturated compounds. The syntheses, structures, and reactions of this new class of substances will be summarized.     

Chemistry of Metallacyclobutadienes

Metallacyclobutadienes are analogues of cyclobutadienes in which one of the cyclobutadiene CR groups has been formally replaced by a transition‐metal fragment. These metallacycles are interesting because they can play an important role in catalysis and can serve as starting materials for the syntheses of organometallic compounds such as metallabenzene, η⁵‐cyclopentadienyl, and η³‐cyclopropenyl complexes. Unlike cyclobutadienes, metallacyclobutadienes can be significantly more stable. A number of metallacyclobutadienes have now been isolated and thoroughly characterized, especially for those that contain transition metals of groups 5–9. Their properties have also been actively investigated. This article highlights the chemistry of metallacyclobutadienes with reference to their syntheses, reactivity, and structural properties.     

Synthesis,Structure and Reactivity of Sulfur-Rich Cyclopentadienyl-Transition Metal Complexes: Sulfur Chemistry from an Organometallic Point of View

Metal-sulfur centers play an important role in the activity of metalloproteins in enzymatic catalysis and the activity of metal sulfides as heterogeneous catalysts. The systematic search for M? S model compounds led to the discovery of an interesting and novel structural chemistry, which stems from the numerous coordination possibilities of sulfur ligands. The intention of this review article is to present and outline new approaches to sulfur chemistry from the organometallic point of view. Reactive cyclopentadienyl-transition metal fragments incorporate elemental sulfur to give polynuclear sulfur-rich complexes, which can contain either mono-, di- or polysulfido ligands or several such ligands in combined form. The versatile structural chemistry of the complexes formed and their reactivity towards organic, inorganic and organometallic compounds are discussed, and examples of some simple and rational procedures for their synthesis starting from cyclopentadienylcarbonyl- and cyclopentadienylhydrido-complexes are outlined. Their reactivity is manifested in numerous metal- and ligandcentered reactions. Finally the, albeit far less extensive, complex chemistry of the other chalcogens (O, Se, Te) is also considered for comparison, thus providing a more detailed survey of particular aspects of this area of chemistry.     

Cluster Chemistry

Metal cluster chemistry is one of the most rapidly developing areas of inorganic and organometallic chemistry. Prior to 1960 only a few metal clusters were well characterized. However, shortly after the early development of boron cluster chemistry, the field of metal cluster chemistry began to grow at a very rapid rate and a structural and a qualitative theoretical understanding of clusters came quickly. Analyzed here is the chemistry and the general significance of clusters with particular emphasis on the cluster research within my group. The importance of coordinately unsaturated, very reactive metal clusters is the major subject of discussion.     

Clusters of Valence Electron Poor Metals—Structure,Bonding, and Properties

Metals in low oxidation states are capable of forming metal–metal bonds. An attempt has been made to classify the numerous phases and structures occurring in such metal-rich systems of valence electron poor metals in some sort of order from a rather general point of view. With this purpose in mind, clusters of these elements, their different types of interconnections, and their condensation via shared metal atoms, which finally leads to extended M? M bonded structures, are described. Interstitial atoms play an important role in stabilizing electron deficient clusters, and can actually lead to the loss of all M? M bonds. Surprising similarities emerge between apparently very different systems as the metal-rich oxides of alkali metals, the oxides, halides, and chalcogenides of d transition metals, and the halides and carbide halides of the lanthanoids.     

Structure Optimisation of Large Transition‐Metal Complexes with Extended Tight‐Binding Methods

Large transition‐metal complexes are used in numerous areas of chemistry. Computer‐aided theoretical investigations of such complexes are limited by the sheer size of real systems often consisting of hundreds to thousands of atoms. Accordingly, the development and thorough evaluation of fast semi‐empirical quantum chemistry methods that are universally applicable to a large part of the periodic table is indispensable. Herein, we report on the capability of the recently developed GFNn‐xTB method family for full quantum‐mechanical geometry optimisation of medium to very large transition‐metal complexes and organometallic supramolecular structures. The results for a specially compiled benchmark set of 145 diverse closed‐shell transition‐metal complex structures for all metals up to Hg are presented. Further the GFNn‐xTB methods are tested on three established benchmark sets regarding reaction energies and barrier heights of organometallic reactions.