Thermodynamics of phase transitions in Zintl clusters from density functional theory: making and breaking of bonds in Ba3Ge4

Density functional theory, in conjunction with the quasi-harmonic approximation, has been used to study the equilibrium between the orthorhombic and tetragonal phases of Ba3Ge4. A transition from the high-temperature tetragonal phase containing isolated Ge46- units to the low-temperature orthorhombic phase, where precisely half of the Ge46- units are polymerised along one axis, is predicted at 930 K, somewhat higher than the experimental value of 630 K. An analysis of the phonon density of states shows that the lower entropy of the orthorhombic phase is not associated directly with the polymerisation of the Ge46- units, but rather with the contraction of the unit cell, which raises the frequencies of ion-ion modes involving the relative motions of the Ba2+ and Ge46- units. Calculations also predict that a third, as yet unobserved, p-tetragonal phase, where all of the Ge46- units are polymerised to form two separate chains running in orthogonal directions, might be accessible at pressures close to 1 GPa.

Snap-shots of cluster growth: structure and properties of a Zintl ion with an Fe3 core, [Fe3Sn18]4

The endohedral Zintl-ion cluster [Fe3Sn18]4- contains a linear Fe3 core with short Fe-Fe bond lengths of 2.4300(9) Å. The ground state is a septet, with significant σ and π contributions to the Fe-Fe bonds. The Sn18 cage is made up of two partially fused Sn9 fragments, and is structurally intermediate between [Ni2CdSn18]6-, where the fragments are clearly separated and [Pd2Sn18]4-, where they are completely fused. It therefore represents an intermediate stage in cluster growth. Analysis of the electronic structure suggests that the presence of the linear Fe-Fe-Fe unit is an important factor in directing reactions towards fusion of the two Sn9 units rather than the alternative of oligomerization via exo bond formation.

Synthesis and Characterization of [Fe3 (As3 )3 (As4 )]3- , a Binary Fe/As Zintl Cluster With an Fe3 Core

We report here the synthesis and structural characterization of the first binary iron arsenide cluster anion, [Fe₃ (As₃ )₃ (As₄ )]^3- , present in both [K([2.2.2]crypt)]₃ [Fe₃ (As₃ )₃ (As₄ )] (1) and [K(18-crown-6)]₃ [Fe₃ (As₃ )₃ (As₄ )]⋅en (2). The cluster contains an Fe₃ triangle with three short Fe-Fe bond lengths (2.494(1) Å, 2.459(1) Å and 2.668(2) Å for 1, 2.471(1) Å, 2.473(1) Å and 2.660(1) Å for 2), bridged by a 2-butene-like As₄ unit. An analysis of the electronic structure using DFT reveals a triplet ground state with direct Fe-Fe bonds stabilizing the Fe₃ core.

High- versus Low-Spin Ni2+ in Elongated Octahedral Environments: Sr2NiO2Cu2Se2, Sr2NiO2Cu2S2, and Sr2NiO2Cu2(Se1-x S x )2

Sr₂NiO₂Cu₂Se₂, comprising alternating [Sr₂NiO₂]²⁺ and [Cu₂Se₂]^2- layers, is reported. Powder neutron diffraction shows that the Ni²⁺ ions, which are in a highly elongated NiO₄Se₂ environment with D_4h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × √2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) μ_B per Ni²⁺ ion. The adoption of the high-spin configuration for this d ⁸ cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr₂NiO₂Cu₂S₂, which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni²⁺, and accordingly, there is no evidence for magnetic moment on the Ni²⁺ ions. Examination of the solid solution Sr₂NiO₂Cu₂(Se_1-x S _x )₂ shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr₂NiO₂Cu₂Se₂ up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni²⁺ coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr₂NiO₂Cu₂Se₂ and Sr₂NiO₂Cu₂S₂, suggest that simple high-spin and low-spin models for Ni²⁺ may not be entirely appropriate and point to further complexities in these compounds.

Missing Link in the Growth of Lead-Based Zintl Clusters: Isolation of the Dimeric Plumbaspherene [Cu4Pb22]4

We report here the structure of an endohedral plumbaspherene, [Cu₄Pb₂₂]^4–, the gold analogue of which was previously postulated to be a “missing link” in the growth of larger clusters containing three and four icosahedral subunits. The cluster contains two [Cu₂Pb₁₁]^2– subunits linked through a Cu₂Pb₄ trigonal antiprism. Density functional theory reveals that the striking ability of mixed Pb/coinage metal Zintl clusters to oligomerize and, in the case of Au, to act as a site of nucleation for additional metal atoms, is a direct consequence of their nd¹⁰(n + 1)s⁰ configuration, which generates both a low-lying (n + 1)s-based LUMO and also a high-lying Pb-centered HOMO. Cluster growth and nucleation is then driven by this amphoteric character, allowing the clusters to form donor–acceptor interactions between adjacent icosahedral units or to additional metal atoms.

Sequential Pressure-Induced B1-B2 Transitions in the Anion-Ordered Oxyhydride Ba2YHO3

We present a detailed experimental and computational investigation of the influence of pressure on the mixed-anion oxyhydride phase Ba₂YHO₃, which has recently been shown to support hydride conductivity. The unique feature of this layered perovskite is that the oxide and hydride anions are segregated into distinct regions of the unit cell, in contrast to the disordered arrangement in closely related Ba₂ScHO₃. Density functional theory (DFT) calculations reveal that the application of pressure drives two sequential B1–B2 transitions in the interlayer regions from rock salt to CsCl-type ordering, one in the hydride-rich layer at approximately 10 GPa and another in the oxide-rich layer at 35–40 GPa. To verify the theoretical predictions, we experimentally observe the structural transition at 10 GPa using high-pressure X-ray diffraction (XRD), but the details of the structure cannot be solved due to peak broadening of the XRD patterns. We use DFT to explore the structural impact of pressure on the atomic scale and show how the pressure-dependent properties can be understood in terms of simple electrostatic engineering.

We investigate a sequence of pressure-induced phase transitions in Ba₂YHO₃, a perovskite oxyhydride with a unique layered anion ordering. Density functional theory and X-ray diffraction together provide a detailed and informative picture of the changes to the crystal structure across the pressure range. This work provides new insights into nonuniform structural flexibility in 2D materials, which can aid targeted materials design in other chemical systems.

Evolution of Vibrational Spectra in the Manganese-Silicon Clusters Mn2Sin, n = 10, 12, and 13, and Cationic [Mn2Si13]. J Phys Chem A 2022;126:1617-1626. [PMID: 35238570 PMCID: PMC9084549 DOI: 10.1021/acs.jpca.1c10027]

A comparison of DFT-computed and measured infrared spectra reveals the ground state structures of a series of gas-phase silicon clusters containing a common Mn₂ unit. Mn₂Si₁₂ and [Mn₂Si₁₃]⁺ are both axially symmetric, allowing for a clean separation of the vibrational modes into parallel (a₁) and perpendicular (e₁) components. Information about the Mn–Mn and Mn–Si bonding can be extracted by tracing the evolution of these modes as the cluster increases in size. In [Mn₂Si₁₃]⁺, where the antiprismatic core is capped on both hexagonal faces, a relatively simple spectrum emerges that reflects a pseudo-D_6d geometry. In cases where the cluster is more polar, either because there is no capping atom in the lower face (Mn₂Si₁₂) or the capping atom is present but displaced off the principal axis (Mn₂Si₁₃), the spectra include additional features derived from vibrational modes that are forbidden in the parent antiprism.

Synthesis and Characterization of Ternary Clusters Containing the [As16]10- Anion, [MM'As16]4- (M = Nb or Ta; M' = Cu or Ag)

The [Nb@As₈]^3- anion was first isolated from solution in 1986, and a number of isostructural [M@Pn₈]^n- clusters (M = Nb, Cr, or Mo; Pn = As or Sb; n = 2 or 3) have since been reported. We show here how anions of this class can be used as synthetic precursors that, in combination with sources of low-valent late transition metals (Cu and Ag), generate ternary polyarsenide cluster anions with unprecedented structural motifs. Chain type [MM'As₁₆]^4- (M = Nb or Ta; M' = Cu or Ag) units are found in compounds 2-5. These clusters contain a nortricyclane-like As₇ cage and a [M@As₈] crown, linked by a single As atom, and represent a fusion of two quite distinct branches of polyarsenide chemistry. Our analysis of the electronic structure confirms that the cluster retains many of the features of the component units. Electrospray ionization mass spectrometry reveals a series of smaller component ions containing 8-12 As atoms, the density functional theory-computed structures of which can be understood in terms of the pseudoelement concept. This work not only presents a new type of coordination mode for As clusters but also offers a point of entry for the rational design of multinary arsenic-based materials.

Synthesis and characterisation of the ternary intermetalloid clusters {M@[As8(ZnMes)4]}3– (M = Nb, Ta) from binary [M@As8]3– precursors

The development of rational synthetic routes to inorganic arsenide compounds is an important goal because these materials are finding applications in many areas of materials science. In this paper, we show that the binary crown clusters [M@As₈]³⁻ (M = Nb, Ta) can be used as synthetic precursors which, when combined with ZnMes₂, generate ternary intermetalloid clusters with 12-vertex cages, {M@[As₈(ZnMes)₄]}³⁻ (M = Nb, Ta). Structural studies are complemented by mass spectrometry and an analysis of the electronic structure using DFT. The synthesis of these clusters presents new opportunities for the construction of As-based nanomaterials.

Two ternary intermetalloid clusters were constructed through binary intermetalloid clusters with a low valent group 12 metal salt. These clusters represent the first example of the structural transformation for intermetalloid clusters.

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.

A linear metal-metal bonded tri-iron single-molecule magnet

The linear trinuclear complex cation [Fe₃(DpyF)₄]²⁺ was prepared as [Fe₃(DpyF)₄](BF₄)₂·2CH₃CN. With large Fe-Fe distances of 2.78 Å, this complex demonstrates intramolecular ferromagnetic coupling between the anisotropic Fe^II centers (J/k_B = +20.9(5) K) giving an S_T = 6 ground state and exhibits single-molecule magnet properties.

Open Shells in Endohedral Clusters: Structure and Bonding in the [Fe2@Ge16]4- Anion and Comparison to Isostructural [Co2@Ge16]4

The anionic cluster [Fe₂@Ge₁₆]^4- has been characterized and shown to be isostructural to the known D_2h-symmetric α isomer of the cobalt analogue [Co₂@Ge₁₆]^4-. Together with the known pair of compounds [Co@Ge₁₀]^3- and [Fe@Ge₁₀]^3-, the title compound completes a set of four closely related germanium clusters that allow us to explore how the metal-metal and metal-cage interactions evolve as a function of size and of the identity of the metal. The results of spin-unrestricted density functional theory (DFT) and multiconfigurational self-consistent field (MC-SCF) calculations present a consistent picture of the electronic structure where transfer of electron density from the metal to the cage is significant, particularly in the Fe clusters where the exchange stabilization of unpaired spin density is an important driving force.

Solution-Based Group 14 Zintl Anions: New Frontiers and Discoveries

ConspectusGroup 14 Zintl anions [E_x]^q- (E = Si-Pb, x = 4, 5, 9, 10) are synthetically accessible, and their diverse chemical reactivity makes them valuable synthons in the construction of larger nanoclusters with remarkable structures, intriguing patterns of chemical bonding, and tunable physical and chemical properties. A plethora of novel cluster anions have now been isolated from the reactions of polyanionic [E_x]^q- precursors with low-valent d-/f-block metal complexes, main-group organometallics, or organics in polar aprotic solvents. The range of products includes intermetalloid clusters with transition metal atom(s) embedded in main-group element cages, organometallic Zintl anions in which [E_x]^q- acts as a ligand, intermetallic Zintl anions where [E_x]^q- is bridged by ligand-free transition metal atom(s), organo-Zintl anions where [E_x]^q- is functionalized with organic-group(s), and oligomers formed through oxidative coupling reactions. The synthesis and characterization of these unconventional complexes, where important contributions to stability come from ionic, covalent, and metal-metal bonds as well as weaker aurophilic and van der Waals interactions, extend the boundaries of coordination chemistry and solid-state chemistry. Substantial progress has been made in this field over the past two decades, but there are still many mysteries to unravel related to the cluster growth mechanism and the controllable synthesis of targeted clusters, along with the remarkable and diverse patterns of chemical bonding that present a substantial challenge to theory. In this Account, we hope to shed some light on the relationship between structure, electronic properties, and cluster growth by highlighting selected examples from our recent work on homoatomic deltahedral [E_x]^q- anions, including (1) germanium-based Zintl clusters, such as the supertetrahedral intermetallic clusters [M₆Ge₁₆]^4- (M = Zn, Cd) and the sandwich cluster {(Ge₉)₂[η⁶-Ge(PdPPh₃)₃]}^4- with a heterometallic Ge@Pd₃ interlayer; (2) tin-based intermetalloid clusters [M_x@Sn_y]^q- and the application of [Co@Sn9]^4- in bottom-up synthesis; and (3) lead clusters with precious metal cores, including the largest Zintl anion [Au₁₂Pb₄₄]^8-. In addition to their intrinsic appeal from a structural and electronic perspective, these new cluster anions also show promise as precursors for the development of new materials with applications in heterogeneous catalysis, where we have recently reported the selective reduction of CO₂.

Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

We describe the reactivity of two linkage isomers of a boryl-phosphaethynolate, [B]OCP and [B]PCO (where [B]=N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), towards tris- (pentafluorophenyl)borane (BCF). These reactions afforded three constitutional isomers all of which contain a phosphaalkene core. [B]OCP reacts with BCF through a 1,2 carboboration reaction to afford a novel phosphaalkene, E-[B]O{(C6 F5 )2 B}C=P(C6 F5 ), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C6 F5 )2 B}(C6 F5 )C=PO[B]. By contrast, [B]PCO undergoes a 1,3-carboboration process accompanied by migration of the N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl to the carbon centre.

A family of lead clusters with precious metal cores

Gold nanoparticles have been used for centuries, both for decoration and in medical applications. More recently, many of the major advances in cluster chemistry have involved well-defined clusters containing tens or hundreds of atoms, either with or without a ligand shell. In this paper we report the synthesis of two gold/lead clusters, [Au₈Pb₃₃]⁶⁻ and [Au₁₂Pb₄₄]⁸⁻, both of which contain nido [Au@Pb₁₁]³⁻ icosahedra surrounding a core of Au atoms. Analogues of these large clusters are not found in the corresponding Ag chemistry: instead, the Ag-centered nido icosahedron, [Ag@Pb₁₁]³⁻, is the only isolated product. The structural chemistry, along with the mass spectrometry which shows the existence of [Au₂Pb₁₁]²⁻ but not [Ag₂Pb₁₁]²⁻, leads us to propose that the former species is the key intermediate in the growth of the larger clusters. Density functional theory indicates that secondary π-type interactions between the [Au@Pb₁₁]³⁻ ligands and the gold core play a significant part in stabilizing the larger clusters.

Many Zintl ions with a single endohedrally encapsulated transition metal ion are known, but relatively few where clusters of two or more metals are present. Here, the authors report the synthesis and characterization of two clusters, [Au₈Pb₃₃]⁶⁻ and [Au₁₂Pb₄₄]¹²⁻, which contain Au₈ and Au₁₂ cores surrounded by Pb shells.

Structural, spectroscopic and electronic properties of a family of face-shared bi-octahedral Ru25+/6+ complexes with a bridging 2,5-di(2-pyridyl)pyrrolide ligand

Herein, we found that nitrogen donor dpp⁻ligand is flexible and provides a platform for aggregation of Ru²⁺ and/or Ru³⁺ to give a new type of face-sharing bi-octahedra Ru₂⁵⁺ and Ru₂⁶⁺ dimer featuring of distinct metal–metal bond bond between two ruthenium centres.

Base induced isomerisation of a phosphaethynolato-borane: mechanistic insights into boryl migration and decarbonylation to afford a triplet phosphinidene

We report on the (tert-butyl)isocyanide-catalysed isomersation of a phosphaethynolato-borane, [B]OCP ([B] = N,N′-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), to its linkage isomer, a phosphaketenyl-borane, [B]PCO. Mechanistic insight into this unusual isomerisation was gained through a series of stoichiometric reactions of [B]OCP with isocyanides and theoretical calculations at the Density Functional Theory (DFT) level. [B]PCO decarbonylates under photolytic conditions to afford a novel boryl-substituted diphosphene, [B]P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> P[B]. This reaction proceeds via a transient triplet phosphinidene which we have been able to observe spectroscopically by Electron Paramagnetic Resonance (EPR) spectroscopy.

We report on the (tert-butyl)isocyanide-catalysed isomersation of a phosphaethynolato-borane, [B]OCP, to its linkage isomer, a phosphaketenyl-borane, [B]PCO.

Pressure-Induced Transitions in the 1-Dimensional Vanadium Oxyhydrides Sr2VO3H and Sr3V2O5H2, and Comparison to 2-Dimensional SrVO2H

High-pressure X-ray diffraction measurements on the layered oxyhydrides Sr₂VO₃H and Sr₃V₂O₅H₂ reveal that both compounds undergo a pressure-induced rock-salt to CsCl (B1-B2) structural transition, similar to those observed in binary compounds (oxides, halides, chalcogenides, etc.). This structural transition, observed at 43 and 45 GPa in Sr₂VO₃H and Sr₃V₂O₅H₂, respectively, relieves almost all of the accumulated strain on the infinite V-O-V ladders, such that the V-O bond lengths are almost identical at 0 and 50 GPa but are substantially compressed at intermediate pressures. The resistances of both materials with 1-dimensional VO ladders decrease with increasing pressure, but unlike SrVO₂H that contains 2-dimensional VO₂ sheets, they remain insulating even at the highest accessible pressures. The reduction in dimensionality from planar to linear VO networks reduces the dispersion of the V-O π bands that define the band gap and leads to insulating behavior at all measured pressures.

Synthesis and structure of a family of rhodium polystannide clusters [Rh@Sn10]3-, [Rh@Sn12]3-, [Rh2@Sn17]6- and the first triply-fused stannide, [Rh3@Sn24]5

Through relatively subtle changes in reaction conditions, we have been able to isolate four distinct Rh/Sn cluster compounds, [Rh@Sn10]3-, [Rh@Sn12]3-, [Rh2@Sn17]6- and [Rh3@Sn24]5-, from the reaction of K4Sn9 with [(COE)2Rh(μ-Cl)]2(COE = cyclooctene). The last of these has a hitherto unknown molecular topology, an edge-fused polyhedron containing three Rh@Sn10 subunits, and represents the largest endohedral Group 14 Zintl cluster yet to have been isolated from solution. DFT has been used to place these new species in the context of known cluster chemistry. ESI-MS experiments on the reaction mixtures reveal the ubiquitous presence of {RhSn8} fragments that may play a role in cluster growth.

B(C6 F5 )3 -Enabled Synthesis of a Cyclic cis-Arsaphosphene

The synthesis and characterization of an (arsino)phosphaketene, As(PCO){[N(Dipp)](CH₂ )}₂ (Dipp=2,6-diisopropylphenyl) is reported along with its subsequent reactivity with B(C₆ F₅ )₃ . When reacted in a stoichiometric ratio, B(C₆ F₅ )₃ drove the insertion of the P=C bond of the phosphaketene into one of the As-N bonds of the arsino functionality, affording an acid-stabilized, seven-membered, cyclic arsaphosphene. In contrast, when catalytic amounts of B(C₆ F₅ )₃ were employed, dimeric species, which formed through a formal [2+2] cycloaddition of the cyclic arsaphosphene, were generated. The cyclic arsaphosphene product represents the first example of such a compound in which the two substituents are arranged in a cis-configuration.

Structural isomerism in the [(Ni@Sn9)In(Ni@Sn9)]5− Zintl ion

A new Zintl cluster, [(Ni@Sn₉)In(Ni@Sn₉)]⁵⁻, has been isolated in two distinct isomeric forms, one where both Ni@Sn₉ units are coordinated to the bridging indium atom in an η³- mode, the other where one is η³- and the other η⁴-.

Quantum chemical models for the absorption of endohedral clusters on Si(111)-(7 × 7): a subtle balance between W–Si and Si–Si bonding

The absorption of endohedral clusters on Si(111)-7 × 7 generates a new bond between W and a surface silicon adatom.

Synthesis, Structure, and Properties of the Layered Oxide Chalcogenides Sr2CuO2Cu2S2 and Sr2CuO2Cu2Se2

The structures of two new oxide chalcogenide phases, Sr₂CuO₂Cu₂S₂ and Sr₂CuO₂Cu₂Se₂, are reported, both of which contain infinite CuO₂ planes containing Cu²⁺ and which have Cu⁺ ions in the sulfide or selenide layers. Powder neutron diffraction measurements show that Sr₂CuO₂Cu₂Se₂ exhibits long-range magnetic ordering with a magnetic structure based on antiferromagnetic interactions between nearest-neighbor Cu²⁺ ions, leading to a √2 a × √2 a × 2 c expansion of the nuclear cell. The ordered moment of 0.39(6) μ_B on the Cu²⁺ ions at 1.7 K is consistent with the value predicted by density functional theory calculations. The compounds are structurally related to the cuprate superconductors and may also be considered as analogues of the parent phases of this class of superconductor such as Sr₂CuO₂Cl₂ or La₂CuO₄. In the present case, however, the top of the chalcogenide-based valence band is very close to the vacant Cu²⁺ 3d states of the conduction band, leading to relatively high measured conductivity.

LaSr3 NiRuO4 H4 : A 4d Transition-Metal Oxide-Hydride Containing Metal Hydride Sheets

The synthesis of the first 4d transition metal oxide-hydride, LaSr₃ NiRuO₄ H₄ , is prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands, a unique structural feature with obvious parallels to the CuO₂ sheets present in the superconducting cuprates. DFT calculations confirm the presence of S=1/2  Ni⁺ and S=0, Ru²⁺ centers, but neutron diffraction and μSR data show no evidence for long-range magnetic order between the Ni centers down to 1.8 K. The observed weak inter-cation magnetic coupling can be attributed to poor overlap between Ni 3dz2 and H 1s in the super-exchange pathways.

Oxidative Addition, Transmetalation, and Reductive Elimination at a 2,2'-Bipyridyl-Ligated Gold Center

Three-coordinate bipyridyl complexes of gold, [(κ²-bipy)Au(η²-C₂H₄)][NTf₂], are readily accessed by direct reaction of 2,2'-bipyridine (bipy), or its derivatives, with the homoleptic gold ethylene complex [Au(C₂H₄)₃][NTf₂]. The cheap and readily available bipyridyl ligands facilitate oxidative addition of aryl iodides to the Au(I) center to give [(κ²-bipy)Au(Ar)I][NTf₂], which undergo first aryl-zinc transmetalation and second C-C reductive elimination to produce biaryl products. The products of each distinct step have been characterized. Computational techniques are used to probe the mechanism of the oxidative addition step, offering insight into both the origin of the reversibility of this process and the observation that electron-rich aryl iodides add faster than electron-poor substrates. Thus, for the first time, all steps that are characteristic of a conventional intermolecular Pd(0)-catalyzed biaryl synthesis are demonstrated from a common monometallic Au complex and in the absence of directing groups.

The structural landscape in 14-vertex clusters of silicon, M@Si14: when two bonding paradigms collide

The structural chemistry of the title clusters has been the source of controversy in the computational literature because the identity of the most stable structure appears to be pathologically dependent on the chosen theoretical model. The candidate structures include a D_3h-symmetric 'fullerene-like' isomer with 3-connected vertices (A), an 'arachno' architecture (B) and an octahedral isomer with high vertex connectivities typical of 'closo' electron-deficient clusters (C). The key to understanding these apparently very different structures is the fact that they make use of the limited electron density available from the endohedral metal in very different ways. Early in the transition series the favoured structure is the one that maximises transfer of electron density from the electropositive metal to the cage whereas for later metals it is the one that minimises repulsions with the increasingly core-like d electrons. The varying role of the d electrons across the transition series leads directly to strong functional dependency, and hence to the controversy in the literature.

Temperature dependence of the spin state and geometry in tricobalt paddlewheel complexes with halide axial ligands

A series of tricobalt EMACs with halide axial ligands demonstrate diverse spin-crossover behaviors as a function of the molecular geometry and the nature of the axial ligand.

Site preferences in hetero-metallic [Fe9-xNix] clusters: a combined crystallographic, spectroscopic and theoretical analysis

The reaction of mixtures of Fe(O₂CMe)₂·2H₂O and Ni(O₂CMe)₂·4H₂O of various compositions with di-2-pyridyl ketone (py₂CO, dpk) in MeCN under an inert atmosphere afforded a family of hetero-metallic enneanuclear clusters with general formula [Fe_9-xNi_x(μ₄-OH)₂(O₂CMe)₈(py₂CO₂)₄] (2, x = 1.00; 3: x = 6.02; 4, x = 7.46; 5, x = 7.81). Clusters 2-5 are isomorphous to the homo-metallic [Fe₉] cluster (1) previously reported by some of us, and also isostructural to the known homo-metallic [Ni₉] cluster. All four clusters contain a central M^II ion in an unusual 8-coordinate site and eight peripheral M^II ions in distorted octahedral environments. The distribution of Fe^II and Ni^II ions over these two distinct coordination sites in 2-5 can be established through a combination of X-ray fluorescence and Mössbauer spectroscopies, which show that Fe^II preferentially occupies the unique 8-coordinate metal site while Ni^II accumulates in the octahedral holes. Density functional theory indicates that the distribution of ions across the two sites arises not from any intrinsic preference of the Fe^II ions for the 8-coordinate sites, but rather because of the large ligand field stabilization energy available to Ni^II in octahedral coordination.

A Redox-Induced Spin-State Cascade in a Mixed-Valent Fe3 (μ3 -O) Triangle

One-electron reduction of a pyrazolate-bridged triangular Fe₃ (μ₃ -O) core induces a cascade wherein all three metal centers switch from high-spin Fe³⁺ to low-spin Fe^2.66+ . This hypothesis is supported by spectroscopic data (¹ H-NMR, UV-vis-NIR, infra-red, ⁵⁷ Fe-Mössbauer, EPR), X-ray crystallographic characterization of the cluster in both oxidation states and also density functional theory. The reduction induces substantial contraction in all bond lengths around the metal centers, along with diagnostic shifts in the spectroscopic parameters. This is, to the best of our knowledge, the first example of a one-electron redox event causing concerted change in multiple iron centers.

Probing the Structure, Dynamics, and Bonding of Coinage Metal Complexes of White Phosphorus

A series of cationic white phosphorus complexes of the coinage metals Au and Cu have been synthesised and characterised both in the solid state and in solution. All complexes feature a P4 unit coordinated through an edge P-P vector (η(2)-like), although the degree of activation (as measured by the coordinated P-P bond length) is greater in the gold species. All of the cations are fluxional on the NMR timescale at room temperature, but in the case of the gold systems fluxionality is frozen out at -90 °C. Electronic structure calculations suggest that this fluxionality proceeds via an η(1)-coordinated M-P4 intermediate.

Ambient-Temperature Synthesis of 2-Phosphathioethynolate, PCS-, and the Ligand Properties of ECX- (E = N, P; X = O, S)

A synthesis of the 2-phosphathioethynolate anion, PCS-, under ambient conditions is reported. The coordination chemistry of PCO-, PCS- and their nitrogen-containing congeners is also explored. Photolysis of a solution of W(CO)6 in the presence of PCO- [or a simple ligand displacement reaction using W(CO)5(MeCN)] affords [W(CO)5(PCO)]- (1). The cyanate and thiocyanate analogues, [W(CO)5(NCO)]- (2) and [W(CO)5(NCS)]- (3), are also synthesised using a similar methodology, allowing for an in-depth study of the bonding properties of this family of related ligands. Our studies reveal that, in the coordination sphere of tungsten(0), the PCO- anion preferentially binds through the phosphorus atom in a strongly bent fashion, while NCO- and NCS- coordinate linearly through the nitrogen atom. Reactions between PCS- and W(CO)5(MeCN) similarly afford [W(CO)5(PCS)]-; however, due to the ambidentate nature of the anion, a mixture of both the phosphorus- and sulfur-bonded complexes (4a and 4b, respectively) is obtained. It was possible to establish that, as with PCO-, the PCS- ion also coordinates to the metal centre in a bent fashion.

Biradical character in the ground state of [Mn@Si12]+: a DFT and CASPT2 study

DFT and CASPT2 calculations reveal that the ground state of [Mn@Si₁₂]⁺is a biradical, quite unlike isoelectronic and isostructural Cr@Si₁₂.

On the structural landscape in endohedral silicon and germanium clusters, M@Si12 and M@Ge12

Amongst the endohedral clusters of the tetrel elements, M@En, the 12-vertex species are unique in that three completely different geometries, the icosahedron (Ih, [Ni@Pb12](2-)), the hexagonal prism (HP, Cr@Si12) and the bicapped pentagonal prism (BPP, [Ru@Ge12](3-)) have been identified in stable molecules. We explore here the origins of this structural diversity by comparing stability patterns across isovalent and isoelectronic series, M@Si12, M@Ge12 and [M@Ge12](3-). The BPP structure dominates the structural landscape for high valence electron counts (57-60) while the HP has a rather narrower window of stability around the 54-56 count. Moreover the preference for an HP structure is unique to silicon: in no case is a rigorously D6h-symmetric structure the global minimum for M@Ge12. Distortions from the high-symmetry limits, where present, can be traced to degeneracies or near-degeneracies in the frontier orbital domains. In all cases the structure adopted is that which maximizes the delocalization of electron density between the metal and the cluster cage, such that both components attain stable electronic configurations.