Structures and Properties of Metalloid Al and Ga Clusters Open Our Eyes to the Diversity and Complexity of Fundamental Chemical and Physical Processes during Formation and Dissolution of Metals

Supramolecular trapping of a cationic all-metal σ-aromatic {Bi4} ring

Aromaticity in organic molecules is well defined, but its role in metal-only rings remains controversial. Here we introduce a supramolecular stabilization approach of a cationic {Bi4} rhomboid within the symmetric charge sphere of two bowl-shaped dianionic calix[4]pyrrolato indinates. Crystallographic and spectroscopic characterization, quantum chemical analysis and magnetically induced ring currents indicate σ-aromaticity in the formally tetracationic 16-valence electron [Bi4]4+ ring. Computational screening for other p-block elements identifies the planar rhomboid as the globally preferred structure for 16-valence electron four-atomic clusters. The aromatic [Bi4]4+ is isoelectronic to the [Al4]4-, a motif previously observed as antiaromatic in Li3[Al4]- in the gas phase. Thus, subtle factors such as charge isotropy seem to decide over aromaticity or antiaromaticity, advising for caution in debates based on the Hückel model-a concept valid for second-row elements but less deterministic for the heavier congeners.

Pd8(PDip)6: Cubic, Unsaturated, Zerovalent

Atomically precise nanoclusters hold promise for supramolecular assembly and (opto)electronic- as well as magnetic materials. Herein, this work reports that treating palladium(0) precursors with a triphosphirane affords strongly colored Pd8(PDip)6 that is fully characterized by mass spectrometry, heteronuclear and Cross-Polarization Magic-Angle Spinning (CP-MAS) NMR-, infrared (IR), UV-vis, and X-ray photoelectron (XP) spectroscopies, single-crystal X-Ray diffraction (sc-XRD), mass spectrometry, and cyclovoltammetry (CV). This coordinatively unsaturated 104-electron Pd(0) cluster features a cubic Pd8-core, µ4-capping phosphinidene ligands, and is air-stable. Quantum chemical calculations provide insight to the cluster's electronic structure and suggest 5s/4d orbital mixing as well as minor Pd─P covalency. Trapping experiments reveal that cluster growth proceeds via insertion of Pd(0) into the triphosphirane. The unsaturated cluster senses ethylene and binds isocyanides, which triggers the rearrangement to a tetrahedral structure with a reduced frontier orbital energy gap. These experiments demonstrate facile cluster manipulation and highlight non-destructive cluster rearrangement as is required for supramolecular assembly.

Recent advances in atomic cluster synthesis: a perspective from chemical elements

Despite its potential significance, "cluster chemistry" remains a somewhat marginalized topic within the chemistry field. However, atomic clusters with their unusual and unique structures and properties represent a novel material group situated between molecules and nanoparticles or solid matter, judging from both scientific standpoints and historical backgrounds. Surveying an entire material group, including all substances that can be regarded as a cluster, is essential for establishing cluster chemistry as a more prominent chemistry field. This review aims to provide a comprehensive understanding by categorizing, summarizing, and reviewing clusters, focusing on their constituent elements in the periodic table. However, because numerous disparate synthetic processes have been individually developed to date, their straightforward and uniform classification is a challenging task. As such, comprehensively reviewing this field from a chemical composition viewpoint presents significant obstacles. It should be therefore noted that despite adopting a synthetic method-based classification in this review, the discussions presented herein could entail inaccuracies. Nevertheless, this unorthodox viewpoint unfolds a new scientific perspective which accentuates the common ground between different development processes by emphasizing the lack of a definitive border between their synthetic methods and material groups, thus opening new avenues for cementing cluster chemistry as an attractive chemistry field.

Navigating the Potential Energy Surface of CdSe Magic-Sized Clusters: Synthesis and Interconversion of Atomically Precise Nanocrystal Polymorphs

Magic-sized clusters (MSCs) are kinetically stable, atomically precise intermediates along the quantum dot (QD) reaction potential energy surface. Literature precedent establishes two classes of cadmium selenide MSCs with QD-like inorganic cores: one class is proposed to be cation-rich with a zincblende crystal structure, while the other is proposed to be stoichiometric with a "wurtzite-like" core. However, the wide range of synthetic protocols used to access MSCs has made direct comparisons of their structure and surface chemistry difficult. Furthermore, the physical and chemical relationships between MSC polymorphs are yet to be established. Here, we demonstrate that both cation-rich and stoichiometric CdSe MSCs can be synthesized from identical reagents and can be interconverted through the addition of either excess cadmium or selenium precursor. The structural and compositional differences between these two polymorphs are contrasted using a combination of 1H NMR spectroscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, inductively coupled plasma optical emission spectroscopy, and UV-vis transient absorption spectroscopy. The subsequent polymorph interconversion reactions are monitored by UV-vis absorption spectroscopy, with evidence for an altered cluster atomic structure observed by powder XRD and PDF analysis. This work helps to simplify the complex picture of the CdSe nanocrystal landscape and provides a method to explore structure-property relationships in colloidal semiconductors through atomically precise synthesis.

Three-Membered Aluminacycles

Three-membered-ring scaffolds of carbocycles, namely, cyclopropanes and cyclopropenes, are ubiquitous in natural products and pharmaceutical molecules. These molecules exhibit a peculiar reactivity, and their applications as synthetic intermediates and versatile building blocks in organic synthesis have been extensively studied over the past century. The incorporation of heteroatoms into three-membered cyclic structures has attracted significant attention, reflecting fundamental differences in their electronic/geometric structures and reactivities compared to their carbon congeners and their associated potential for exploitation in applications. Recently, the chemistry of low-valent aluminum species, alumylenes, dialumenes, and aluminyl anions, has dramatically developed, which has allowed access to hitherto unprecedented aluminacycles. This Perspective focuses upon advances in the chemistry of three-membered aluminacycles, including their synthetic protocols, spectroscopic and structural properties, and reactivity toward various substrates and small molecules.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Electronic structure and bonding in endohedral Zintl clusters

Endohedral Zintl clusters-multi-metallic anionic molecules in which a d-block or f-block metal atom is enclosed by p-block (semi)metal atoms-are very topical in contemporary inorganic chemistry. Not only do they provide insight into the embryonic states of intermetallic compounds and show promise in catalytic applications, they also shed light on the nature of chemical bonding between metal atoms. Over the past two decades, a plethora of endohedral Zintl clusters have been synthesized, revealing a fascinating diversity of molecular architectures. Many different perspectives on the bonding in them have emerged in the literature, sometimes complementary and sometimes conflicting, and there has been no concerted effort to classify the entire family based on a small number of unifying principles. A closer look, however, reveals distinct patterns in structure and bonding that reflect the extent to which valence electrons are shared between the endohedral atom and the cluster shell. We show that there is a much more uniform relationship between the total valence electron count and the structure and bonding patterns of these clusters than previously anticipated. All of the p-block (semi)metal shells can be placed on a ladder of total valence electron count that ranges between 4n+2 (closo deltahedra), 5n (closed, three-bonded polyhedra) and 6n (crown-like structures). Although some structural isomerism can occur for a given electron count, the presence of a central metal cation imposes a preference for rather regular and approximately spherical structures which maximise electrostatic interactions between the metal and the shell. In cases where the endohedral metal has relatively accessible valence electrons (from the d or f shells), it can also contribute its valence electrons to the total electron count of the cluster shell, raising the effective electron count and often altering the structural preferences. The electronic situation in any given cluster is considered from different perspectives, some more physical and some more chemical, in a way that highlights the important point that, in the end, they explain the same situation. This article provides a unifying perspective of bonding that captures the structural diversity across this diverse family of multimetallic clusters.

Higher stability of metalloid tin clusters obtained via the cation-anion interaction

The reaction of a metastable SnCl solution with KR (R = Si(SiMe₃)₂(SitBuMe₂) = Hyp^tBuMe₂ or Si(SiMe₃)₂(SiEt₃) = Hyp^Et₃) gives the metalloid tin cluster [Sn₁₀R₄]^2- in partly good yields. The tin clusters are in the solid state as well in solution coordinated by a potassium cation, leading first of all to a more static compound and novel coordination polymers in the solid state. Additionally, the coordination of the potassium cation strongly increases the stability of the cluster in solution. This higher stability is thereby an important prerequisite for future investigation of this open shell metalloid tin cluster.

Structural Chemistry of Giant Metal Based Supramolecules

The review presents a bird-eye view on the state of research in the field of giant nonbiological discrete metal complexes and ions of nanometer size, which are structurally characterized by means of single-crystal X-ray diffraction, using the crystal structure as a common key feature. The discussion is focused on the main structural features of the metal clusters, the clusters containing compact metal oxide/hydroxide/chalcogenide core, ligand-based metal-organic cages, and supramolecules as well as on the aspects related to the packing of the molecules or ions in the crystal and the methodological aspects of the single-crystal neutron and X-ray diffraction of these compounds.

Intermetallic phases meet intermetalloid clusters

In this article intermetalloid clusters of Cu-Zn, Cu-AI, Cu-Sn, and Cu-Pb are discussed. Intermetallic compounds based on these metal combinations are of the Hume-Rothery type with well-defined structures related to the valence electron count of the involved metals. Many Zintl-type and molecular clusters with these metals are known with remarkable structural parallels to the respective solid-state phases. On several examples, this article discusses intermetalloid clusters in terms of their metal core structures and relates them to structural principles in intermetallic solid-state phases. Also the syntheses of such clusters are addressed. Zintl-type and molecular clusters are most generally accessible from organometallic precursor complexes with redox processes between the different metals as an underlying synthesis concept.

Precise Implantation of an Archimedean Ag@Cu12 Cuboctahedron into a Platonic Cu4Bis(diphenylphosphino)hexane6 Tetrahedron

Precision loading of nanoclusters in confined spaces, which has been enthusiastically pursued in the scientific realm, is still associated with some mysteries of "how", "when", and "why". Here, we isolated two similar heterometallic cluster-in-cage compounds, [Ag@Cu₁₂S₈@Cu₄(dpph)₆]X (X = OH, SD/AgCu16a and X = PF₆, SD/AgCu16b; SD = SunDi), by use of an antigalvanic reaction between organometallic [PhC≡CCu]_n and Ph₃CSH with elemental silver. Both compounds are formed by fitting an Archimedean Ag@Cu₁₂ cuboctahedral cluster into a Platonic Cu₄(dpph)₆ tetrahedral cage [dpph = bis(diphenylphosphino)hexane]. The Ag@Cu₁₂ cluster is a hollow cuboctahedral Cu₁₂ cage filled with a central Ag^I atom, and all eight triangular faces of the Ag@Cu₁₂ cuboctahedron are triply capped by eight S^2- ions, four of which in a tetrahedral array further internally pillar four Cu vertices of the outer Cu₄(dpph)₆ tetrahedron, fixing the cluster in the cage. Both compounds can be deemed as molecular fragments excised from porous nanomaterials filled with discrete nanoclusters, thus providing more details for understanding the confined growth of atomically precise nanoclusters. Electrospray ionization mass spectrometry (ESI-MS) reveals that the AgCu₁₆ cluster is quite stable in CH₂Cl₂ and can stepwise lose dpph ligand in the gas phase under increased collision energy. This work not only presents a precise aggregation of metal atoms in a confined cavity to form a cluster-in-cage compound but also provides deep insights into the binding and geometry matching between clusters and cages in one entity.

Modern cluster design based on experiment and theory

For decades, chemists have explored cluster compounds according to theoretical models that have proved too simplistic to accurately predict cluster properties, stabilities and functions. By incorporating molecular symmetry into existing cluster models, we can better study real polyatomic molecules and have new guidelines for their design. This symmetry-adapted cluster model allows us to discover substances that shatter the conventional notion of clusters. Theoretical predictors will point to the viability of new clusters, whose syntheses can be realized with parallel advances in experimental methods. This Perspective describes these modern experimental and theoretical strategies for cluster design and how they may give rise to new fields in cluster chemistry.

Insights into the Composition and Structural Chemistry of Gallium(I) Triflate

"GaOTf" is a simple, convenient source of low-valent gallium for synthetic chemistry and catalysis. However, little is currently known about its composition or reactivity. In this work, 71 Ga NMR spectroscopy shows the presence of [Ga(arene)n ]+ salts on oxidation of Ga metal with AgOTf in arene solvents. However, a more complex picture of speciation is uncovered by X-ray diffraction studies. In all cases, mixed-valence compounds containing Ga-arene and Ga-OTf coordination motifs, in addition to an unusual "naked" [Ga]+ ion, are found. Addition of 18-crown-6 allows for the isolation of a discrete GaI crown complex. Evidence of a potential intermediate in the formation of "GaOTf" has been isolated in the form of the bimetallic silver(I)/gallium(I) cluster anion [Ag4 {Ga(OTf)3 }4 (μ-Ga)6 (OTf)4 ]2- .

An oxygen-passivated vanadium cluster [V@V10O15]− with metal–metal coordination produced by reacting Vn− with O2

An oxygen-passivated vanadium cluster [V@V₁₀O₁₅]⁻ is reported by reacting V_n⁻ with O₂, giving rise to superatom features of metal–metal coordination and 3D aromaticity.

Ligand-protected gold/silver superatoms: current status and emerging trends

Monolayer-protected gold/silver clusters have attracted much interest as nano-scale building units for novel functional materials owing to their nonbulk-like structures and size-specific properties. They can be viewed as ligand-protected superatoms because their magic stabilities and fundamental properties are well explained in the framework of the jellium model. In the last decade, the number of ligand-protected superatoms with atomically-defined structures has been increasing rapidly thanks to the well-established synthesis and structural determination by X-ray crystallography. This perspective summarizes the current status and emerging trends in synthesis and characterization of superatoms. The topics related to synthesis include (1) development of targeted synthesis based on transformation, (2) enhancement of robustness and synthetic yield for practical applications, and (3) development of controlled fusion and assembly of well-defined superatoms to create new properties. New characterization approaches are also introduced such as (1) mass spectrometry and laser spectroscopies in the gas phase, (2) determination of static and dynamic structures, and (3) computational analysis by machine learning. Finally, future challenges and prospects are discussed for further promotion and development of materials science of superatoms.

This perspective summarizes the current status and emerging trends in synthesis and characterization of ligand-protected gold/silver superatoms.

Superatomic Gallium Clusters in Dendrimers: Unique Rigidity and Reactivity Depending on their Atomicity

Superatoms have been investigated due to their possible substitution for other elements. The solution-phase synthesis of superatoms has attracted attention to realize the availability of superatoms. However, the previous approach is basically limited to the formation of a single cluster. Here, superatoms are investigated and the number of valence electrons in these superatoms is changed by designing the number of gallium atoms present. Based on the dendrimer template method, clusters consisting of 3, 12, 13, and other numbers of atoms have been synthesized. The halogen-like superatomic nature of Ga₁₃ is structurally and electrochemically observed as completely different to the other clusters. The gallium clusters of 13 and 3 atoms, which can fill the 2P and 1P superatomic orbitals, respectively, exhibit different reactivities. The 3-atom gallium cluster is suggested as being reduced to Ga₃ H₂ ^- due to the lower shift of energy levels in the unoccupied orbitals. The results for these gallium clusters provide candidates for superatoms.

Iodization Threshold in Size-Dependent Reactions of Lead Clusters Pbn+ with Iodomethane

Utilizing a magnetron-sputtering (MagS) source in tandem with a multiple-ion laminar flow tube (MIFT) reactor and a customized triple quadrupole mass spectrometer (TQMS), we have prepared clean Pb_n⁺ (n = 1-13) clusters and measured their reactivity with iodomethane under high carrier gas pressures. Strong size dependences are found for the reactivity of these cationic Pb_n⁺ clusters with CH₃I. For the Pb_n⁺ with n ≤ 4, iodinated clusters Pb_nI⁺ were found to be the dominant products, in strong contrast to n > 4 where no such products were seen. Quantum chemical studies show that with an increasing number of Pb atoms, the Pb-Pb interatomic interactions become stronger compared with the Pb-I bonding in Pb_nI⁺ clusters. Furthermore, the reactions of Pb_1-4⁺ with CH₃I have fairly small transition state energy barriers, in contrast to those for Pb_n>4⁺ clusters which have magnitudes that will prevent reactions under the ambient conditions.

Multiple-Valence Aluminum and the Electronic and Geometric Structure of Al nO m Clusters

Electronic stability in aluminum clusters is typically associated with either closed electronic shells of delocalized electrons or a +3 oxidation state of aluminum. To investigate whether there are alternative routes toward electronic stability in aluminum oxide clusters, we used theoretical methods to examine the geometric and electronic structure of Al _nO _m (2 ≤ n ≤ 7; 1 ≤ m ≤ 10) clusters. Electronically stable clusters with large HOMO-LUMO (highest occupied molecular orbital and lowest unoccupied molecular orbital) gaps were identified and could be grouped into two categories. (1) Al_{2 n}O_{3 n} clusters with a +3 oxidation state on the aluminum and (2) planar clusters including Al₄O₄, Al₅O₃, Al₆O₅, and Al₆O₆. The structures of the planar clusters have external Al atoms bound to a single O atom. Their electronic stability is explained by the multiple-valence Al sites, with the internal Al atoms having an oxidation state of +3, whereas the external Al atoms have an oxidation state of +1.

Reduction of organic azides by indyl-anions. Isolation and reactivity studies of indium-nitrogen multiple bonds

The synthesis of a new potassium-indyl complex, K[In(NONAr)] (NONAr = [O(SiMe2NAr)2]2-, Ar = 2,6-iPr2C6H3) and its reactivity with organic azides RN3 is reported. When R = 2,6-bis(diphenylmethyl)-4- t Bu-phenyl, a dianionic alkyl-amide ligand is formed via C-H activation across a transient In-Nimide bond. Reducing the size of the R-group to 2,4,6-trimethylphenyl (mesityl, Mes) enables oxidation of the indium and elimination of dinitrogen to afford the imide species, K[In(NONAr)(NMes)]. The anion contains a short In-Nimide bond, shown computationally to contain appreciable multiple bond character. Reaction of isolated imides with an additional equivalent of azide (R = Mes, SiMe3) generates tetrazenido-indium compounds K[In(NONAr){κ-N,N'-N4(Mes)(R)-1,4}], shown by X-ray crystallography to contain planar InN4 heterocycles in the anion.

Sn20(SitBu3)10Cl2 – the largest metalloid group 14 cluster shows a raspberry-like arrangement of smaller units

The reaction of a metastable Sn(i)Cl solution with NaSi^tBu₃ leads to Sn₂₀(Si^tBu₃)₁₀C_l2, where a raspberry-like cluster of cluster arrangement is realized, giving further insight into the formation process of a metalloid tin cluster from molecular precursors.

Synthesis and Reactivity Studies of Amido-Substituted Germanium(I)/Tin(I) Dimers and Clusters

Three amide ligands of varying steric bulk and electronic properties were utilized to prepare a series of amido-germanium(II)/tin(II) halide compounds, (LEX)_n , (L= -N{B(DipNCH)₂ }(SiMe₃ ), ^TBo L; -N{B(DipNCH)₂ }(SiPh₃ ), ^PhBo L; -N(Dip)(tBu), ^DBu L; Dip=C₆ H₃ iPr₂ -2,6; E=Ge or Sn; X=Cl or Br; n=1 or 2). Reductions of these with a magnesium(I) dimer, {(^Mes Nacnac)Mg}₂ (^Mes Nacnac=[(MesNCMe)₂ CH]^- , Mes=mesityl), afforded singly bonded amido-digermynes (^TBo LGe-Ge^TBo L and ^PhBo LGe-Ge^PhBo L), and an amido-distannyne (^PhBo LSn-Sn^PhBo L), in addition to several low-valent, amido stabilized tetrel-tetrel bonded cluster compounds, (^DBu LGe)₄ , (^DBu LSn)₆ and Sn₅ (^TBo L)₄ . The nature of the products resulting from these reactions was largely dependent on the steric bulk of the amide ligand employed. Cluster (^DBu LGe)₄ possessed an unusual folded butterfly structure, the bonding and electronic of which were examined using DFT calculations. Reactions of the amido-germanium(I) compounds with H₂ were explored, and gave rise to the amido-digermene, ^TBo L(H)Ge=Ge(H)^TBo L and the cyclotetragermane, {^DBu L(H)Ge}₄ . Reactions of (^DBu LGe)₄ with a series of unsaturated small molecule substrates yielded ^DBu LGeOGe^DBu L, ^DBu LGe(μ-C₂ H₄ )₂ Ge^DBu L and ^DBu LGe(μ-1,4-C₆ H₈ )(μ-1,2-C₆ H₈ )Ge^DBu L. The latter results imply that (^DBu LGe)₄ can act as a masked source of the digermyne ^DBu LGeGe^DBu L in these reactions. All further reactivity studies indicated that the germanium(I) compounds exhibit a "transition-metal-like" behavior, which is closely related to that previously described for bulky digermynes and related compounds.

Conversion Reactions of Atomically Precise Semiconductor Clusters

Clusters are unique molecular species that can be viewed as a bridge between phases of matter and thus between disciplines of chemistry. The structural and compositional complexity observed in cluster chemistry serves as an inspiration to the material science community and motivates our search for new phases of matter. Moreover, the formation of kinetically persistent cluster molecules as intermediates in the nucleation of crystals makes these materials of great interest for determining and controlling mechanisms of crystal growth. Our lab developed a keen interest in clusters insofar as they relate to the nucleation of nanoscale semiconductors and the modeling of postsynthetic reaction chemistry of colloidal materials. In particular, our discovery of a structurally unique In₃₇P₂₀X₅₁ (X = carboxylate) cluster en route to InP quantum dots has catalyzed our interest in all aspects of cluster conversion, including the use of clusters as precursors to larger nanoscale colloids and as platforms for examining postsynthetic reaction chemistry. This Account is presented in four parts. First, we introduce cluster chemistry in a historical context with a focus on main group, metallic, and semiconductor clusters. We put forward the concept of rational, mechanism-driven design of colloidal semiconductor nanocrystals as the primary motivation for the studies we have undertaken. Second, we describe the role of clusters as intermediates both in the synthesis of well-known material phases and in the discovery of unprecedented nanomaterial structures. The primary distinction between these two approaches is one of kinetics; in the case of well-known phases, we are often operating under high-temperature thermolysis conditions, whereas for materials discovery, we are discovering strategies to template the growth of kinetic phases as dictated by the starting cluster structure. Third, we describe reactions of clusters as model systems for their larger nanomaterial progeny with a primary focus on cation exchange. In the case of InP, cation exchange in larger nanostructures has been challenging due to the covalent nature of the crystal lattice. However, in the higher energy, strained cluster intermediates, cation exchange can be accomplished even at room temperature. This opens opportunities for accessing doped and alloyed nanomaterials using postsynthetically modified clusters as single-source precursors. Finally, we present surface chemistry of clusters as the gateway to subsequent chemistry and reactivity, and as an integral component of cluster structure and stability. Taken as a whole, we hope to make a compelling case for using clusters as a platform for mechanistic investigation and materials discovery.

Embryonic brass: pseudo two electron Cu/Zn clusters

The isoelectronic M₇ clusters [Cu₃Zn₄](Cp*)₅ and {[Cu₂Zn₅](Cp*)₅}⁺ were isolated as unique species pushing the boundaries of the Wade–Mingos rules.

The isoelectronic M₇ clusters [Cu₃Zn₄](Cp*)₅ (1) and {[Cu₂Zn₅](Cp*)₅}⁺ (2) are described. While 1 can be isolated only as a minor side product from the reaction of Cu(CH₃CO₂) with equimolar amounts of [Zn₂Cp*₂] with the trigonal cluster [CuZn₂](Cp*)₃ as the major product, 2 is available in acceptable yields from the reaction of [CuZn₂](Cp*)₃ with the Cp*Zn₂-transfer-reagent [Cp*Zn₂(Et₂O)₃][BAr₄^F]. The trigonal bipyramidal Cu/Zn-clusters exhibit exceptional bonding situations: with formally only one skeleton electron pair they can be regarded as highly electron deficient. However, a detailed DFT analysis reveals that the cluster bonding is supported by 3d orbital contributions of the trigonal metal base unit. The data contribute to the development of an advanced tool-box for synthesis of Hume-Rothery intermetallic (e.g. brass) inspired clusters.

Anion stabilised hypercloso-hexaalane Al6H6

Boron hydride clusters are an extremely diverse compound class, which are of enormous importance to many areas of chemistry. Despite this, stable aluminium hydride analogues of these species have remained staunchly elusive to synthetic chemists. Here, we report that reductions of an amidinato-aluminium(III) hydride complex with magnesium(I) dimers lead to unprecedented examples of stable aluminium(I) hydride complexes, [(ArNacnac)Mg]2[Al6H6(Fiso)2] (ArNacnac = [HC(MeCNAr)2]-, Ar = C6H2Me3-2,4,6 Mes; C6H3Et2-2,6 Dep or C6H3Me2-2,6 Xyl; Fiso = [HC(NDip)2]-, Dip = C6H3Pri2-2,6), which crystallographic and computational studies show to possess near neutral, octahedral hypercloso-hexaalane, Al6H6, cluster cores. The electronically delocalised skeletal bonding in these species is compared to that in the classical borane, [B6H6]2-. Thus, the chemistry of classical polyhedral boranes is extended to stable aluminium hydride clusters for the first time.

Mechanistic Studies of [AlCp*]4 Combustion

The combustion mechanism of [AlCp*]₄ (Cp* = pentamethylcyclopentadienyl), a ligated aluminum(I) cluster, was studied by a combination of experimental and theoretical methods. Two complementary experimental methods, temperature-programmed reaction and T-jump time-of-flight mass spectrometry, were used to investigate the decomposition behaviors of [AlCp*]₄ in both anaerobic and oxidative environments, revealing AlCp* and Al₂OCp* to be the major decomposition products. The observed product distribution and reaction pathways are consistent with the prediction from molecular dynamics simulations and static density functional theory calculations. These studies demonstrated that experiment and theory can indeed serve as complementary and predictive means to study the combustion behaviors of ligated aluminum clusters and may help in engineering stable compounds as candidates for rocket propellants.

An Anionic Aluminium Nucleophile

The first stable anionic aluminium nucleophile was isolated by Goicoechea, Aldridge and co-workers. The aluminyl compound showed very high reactivity in metathesis reactions as well as in the oxidative addition of substrates such as dihydrogen and benzene, which opens up new perspectives in main group chemistry.