The σ2π4 Triple Bond between Molybdenum and Tungsten Atoms: Developing the Chemistry of an Inorganic Functional Group

Numerous analogies between organic and inorganic chemistry have emerged in recent years. The most prominent example is the isolobal relationship. Many reactions have shown that metal-metal double and triple bonds exhibit a pattern of reactivity similar to that of alkenes and alkynes. In compounds containing a σ²π⁴ triple bond between molybdenum and tungsten atoms, the M? M bond order can be increased from three to four by reductive elimination or decreased from three to two or one by oxidative addition. Complexes with M?M bonds can be used to prepare clusters or can serve as catalysts. In this review relationships between structure (electronic and stereochemical) and reactivity that are characteristic for modern inorganic chemistry are discussed.     

Multiple Bonds between Transition Metals and “Bare” Main Group Elements: Links between Inorganic Solid State Chemistry and Organometallic Chemistry

The last two decades have seen a dramatic development in the study of metal-metal multiple bonds, particular successes being recorded in the field of organometallic chemistry. Syntheses designed to produce novel transition metal complexes with single, double, triple and quadruple metal-metal bonds occupy a most important place in such research, as also do reactivity studies. A striving to establish general principles has provided much of the motivation for such work, but one less obvious goal—the commercial application of the catalytic properties of metal-metal multiple bonding systems, in the medium and long term—should not be overlooked. All aspects of the investigations of metal-metal multiple bonds also apply to a particular class of compound that has, however, enjoyed little lime-light and thus deserves the present review: complexes with multiple bonds between transition metals and substituent-free (“bare”) main group elements. Although based mostly on accidental discoveries, the few noteworthy examples are now beginning to unfold general concepts of synthesis that are capable of being extended and thus are deserving of exploitation in preparative chemistry. The availability of further structural patterns exhibiting multiple bonds between transition metals and ligand-free main group elements might enable preparative organometallic chemistry to expand in a completely new direction (for instance by the stabilizing or activation of small molecules at the metal complex). This essay discusses the chemistry of complexes of bare carbon, nitrogen, and oxygen ligands (carbido-, nitrido-, and oxo-complexes) and their relationships to higher homologues from both a synthetic and a structural point of view.     

Trinuclear Clusters of the Early Transition Elements

Trinuclear clusters of the early transition elements, i.e. those elements situated on the left-hand side of the transition series, represent the simplest types of clusters. They characteristically have a pronounced formation tendency and high stability; they are therefore produced under a wide variety of conditions and their triangular M₃ skeleton is conserved in ligand exchange reactions. These clusters play a very important role in the chemistry of the respective elements, and especially those of the 4 d and 5 d series. Biochemical implications are of interest in connection with Mo^IV. This progress report presents a systematic account of the chemistry and the molecular and electronic structure of such compounds. A connection is also established with the crystal field treatment of mononuclear metal complexes.     

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.     

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.     

Monovalent Group 13 Organometallic Compounds: Weak Association to Monomeric,Versatile Two-Electron Donors

A weakly associated hexamer is formed for [GaCp*] (Cp*=C₅Me₅) in the solid state (see picture). The recent X-ray crystal structure analyses of [GaCp*] as well as the monomeric In^I and Tl^I compounds [M(2,4,6-Trip₃C₆H₂)] (Trip=2,4,6-iPr₃C₆H₂) throw new light on the association and aggregation of monovalent Group 13 elements in the solid state. The synthesis of [Ni⁰{In[C(SiMe₃)₃]}₄], a complex with terminally bonded In^IR ligands, offers alternative σ-donor/π-acceptor ligands to organometallic chemists. The newest results in this area are likely to open up new and intriguing possibilities in the preparation of main group–transition metal clusters.     

Synthesis,Structural Characterization and Preliminary Biological Studies of Several Heterocyclic Transition Metal Carbonyl Complexes

The reaction of molybdenum, tungsten and manganese carbonyls with several thiazole heterocycle ligands yielded a number of coordinated transition metal complexes 1 – 10 . Of these complexes 1 – 6 are new compounds which have not been reported to date. The structures of new compounds were characterized by FT‐IR and ¹H‐NMR spectroscopy as well as single‐crystal X‐ray diffraction analysis. Complexes 1 – 10 are carbon monoxide releasing molecules that show structure‐related anti‐cancer activity. The cytotoxicity of all compounds on Hela cells was evaluated by MTT assay, and the results show that carbon monoxide releasing molecules containing such Schiff base ligands may have biomedical applications for their anti‐tumor effect.     

Homoatomic Bonding of Main Group Elements

Clusters of main group elements are not rare. On the contrary, it is becoming difficult to avoid the discovery of new substances of this type. Clusters are the natural intermediate stages between an element and its isolated atoms or ions. In the form of polycations and polyanions they offer models for the stepwise oxidation and reduction of an element and represent a bridge between the elements. The great majority of homonuclear bonded structures are already present in the solid phases of simple systems. Mobilization of these clusters as molecules represents a great challenge.     

Contributions to the Chemistry of the Binary Compounds of the Transition Elements

Phase and structural relationships of the sulfur, selenium, and tellurium compounds of the 4d and 5d transition elements of groups IV to VII of the periodic system are discussed. Homologous elements behave very similarly with respect to the chalcogens, and this is particularly the case for niobium and tantalum, and for molybdenum and tungsten. However, zirconium, niobium, and molybdenum have a greater tendency towards formation of chalcogen-poor phases than their homologues hafnium, tantalum, and tungsten. Subchalcogenides are known only for zirconium and niobium. The number of phases and the tendency towards formation of solid solutions are considerably smaller among the tellurides than among the sulfides and selenides. The crystal structures of the telluride phases also differ from those of the sulfide and selenide phases of analogous composition. In addition, a review of the phase and structural relationships of the arsenic and antimony compounds of the 4d and 5d transition elements of groups V to VII is given.     

New Transition Metal Clusters with Ligands from Main Groups Five and Six

In both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three- or four-membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe₃)₂ (E = S, Se, Te) and E′R(SiMe₃)₂ (R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also possible.     

Coordination Chemistry of Aluminum,Gallium, and Indium at Transition Metals

The current upswing in the interest in organoelement chemistry of Group 13 metals is attributed not least to the establishment of the coordination chemistry of R_aE fragments (E=Al, Ga, In; a=1, 2) at d-block metals (M). Recently the availability of low-valent organoelement compounds as building blocks for synthesis has substantially enriched the structure chemistry of this class of compounds. The M–E bonding conditions and the question of the significance of M(d_π)-E(p_π) backbonding as well as potential applications in materials science, for example, as single-source precursors for the deposition of thin intermetallic films by chemical vapor deposition, are discussed.     

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.     

[Al7{N(SiMe3)2}6]−: A First Step towards Aluminum Metal Formation by Disproportionation

A “naked” aluminum atom links two aluminum tetrahedra in the [Al₇{N(SiMe₃)₂}₆]⁻ ion (see picture), which results from the reaction of a metastable AlCl solution with LiN(SiMe₃)₂ and crystallizes with [Li(OEt₂)₃]⁺ as cation. This unique structure among molecular metal atom clusters represents a small but characteristic section of cubic close-packed aluminum.     

Exploiting Incommensurate Symmetry Numbers: Rational Design and Assembly of M2M3′L6 Supramolecular Clusters with C3h Symmetry

Mixed-metal mesocates [M₂Pd₃Br₆L₆]⁴⁻ (M=Ti^IV, Sn^IV; L=4-diphenylphosphanyl-catecholate) have been synthesized, in which the two incommensurate symmetry elements generated by the different metal ions are linked by a rigid, bifunctional ligand to generate a C_3h-symmetrical cluster (see picture).     

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.     

Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate‐Protected Metal Clusters to Synthesize Controlled Metal Clusters

It is now possible to accurately synthesize thiolate (SR)‐protected gold clusters (Au_n(SR)_m) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Au_n(SR)_m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.     

Gas Phase Ion Chemistry of Transition Metal Clusters: Production, Reactivity, and Catalysis

This review focuses on the use of mass spectrometry to examine the gas phase ion chemistry of metal clusters. Ways of forming gas phase clusters are briefly overviewed and then the gas phase chemistry of silver clusters is discussed to illustrate the concepts of magic numbers and how reactivity can be size dependent. The chemistry of other bare and ligated metal clusters is examined, including mixed metal dimer ions as models for microalloys. Metal clusters that catalyze gas phase chemical reactions such as the oxidation of CO and organic substrates are reviewed. Finally the interface between nanotechnology and mass spectrometry is also considered.     

Organometallic Probe for the Electronics of Base‐Stabilized Group 11 Metal Cations

A number of trimetalloborides have been synthesized through the reactions of base‐stabilized coinage metal chlorides with a dimanganaborylene lithium salt in the hope of using this organometallic platform to compare and evaluate the electronics of these popular coinage metal fragments. The adducts of Cu^I, Ag^I, and Au^I ions, stabilized by tricyclohexylphosphine (PCy₃), N‐1,3‐bis(4‐methylphenyl)imidazol‐2‐ylidene (ITol), or 1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene (CAAC), with [{Cp(CO)₂Mn}₂B]^? were studied spectroscopically, structurally, and computationally. The geometries of the adducts fall into two classes, one symmetric and one asymmetric, each relying on the combined characteristics of both the metal and ligand. The energetic factors proposed as the causes of the structural differences were investigated by ETS‐NOCV (extended transition state‐natural orbitals for chemical valence) analysis, which showed the final geometry to be controlled by the competition between the tendency of the coinage metal to adopt a higher or lower coordination number and the willingness of the cationic fragment to participate in back‐bonding interactions.