[Palladium-decabismuth(4+)]-tetrakis(tetrachloridoaluminate) Cluster Compound, [Pd@Bi₁₀][AlCl₄]₄: Synthesis, Crystal Structure, and Electronic Structure

Abstract

Black, needle-like single crystals of [Pd@Bi₁₀][AlCl₄]₄ were synthesized in a one-pot reaction between PdCl₂, Bi, and BiCl₃ at 180 °C in the Lewis acidic ionic liquid (LAIL) medium [BMIm]Cl∙4.2AlCl₄ (BMIm = 1-n-butyl-3-methylimidazolium). Single-crystal X-ray diffraction revealed that the compound crystallizes in the triclinic space group P$\overline{1}$ with the unit cell parameters a = 11.0233(5) Å, b = 26.1892(14) Å, c = 26.2687(14) Å, α = 90.842(2)°, β = 92.1940(10)°, γ = 91.164(2)°, closely matching its platinum-containing analog. The structure features pentagonal antiprismatic [Pd@Bi₁₀]⁴⁺ cluster cations charge-balanced by tetrahedral [AlCl₄]⁻ anions. Bonding and charge analysis reveal unoptimized Pd–Bi and strong Bi–Bi covalent interactions consistent with electronegativity trends and the previously reported host–guest model. Electronic structure calculations performed with the TB-LMTO-ASA program show that [Pd@Bi₁₀][AlCl₄]₄ exhibits semiconducting behavior, suggesting a bandgap opening of 0.71 eV.

1. Introduction

Halometallate ionic liquids (ILs) are extensively researched room-temperature molten salts formed by the reaction of a metal halide with an organic halide salt [1,2]. These compounds typically melt at temperatures below 100 °C, enhancing the solubility of various reactants in ILs and promoting reactions at low temperatures, which, in turn, enables green chemical processes and provides a uniform reaction environment [3]. Although ILs have long been applied in organic chemistry, their intensive use in inorganic synthesis emerged in the early 21st century, allowing the discovery of numerous inorganic compounds and materials that were inaccessible by traditional synthetic methods such as high-temperature solid-state and solvothermal reactions [4]. ILs possess unique properties that distinguish them from conventional organic solvents, classic melts, or solid-state metal flux systems. These include a wide liquidus range, high redox and thermal stability, negligible vapor pressure, and adjustable polarity, to name a few [1]. Consequently, ILs have found uses in lubrication, electrodeposition, electrolytes in photovoltaic devices, clean catalysis, polymerization, the general synthesis of novel inorganic compounds, and, most notably, crystal engineering of diverse inorganic materials [5,6,7,8,9,10].

Over the past two decades, bismuth cluster compounds have been extensively studied due to their non-toxic and non-radioactive nature, as well as their significant potential as catalysts for novel pharmaceuticals and fine-chemical synthesis. ILs hold great promise for advancing bismuth chemistry, offering low-temperature routes for the synthesis of Bi-bearing clusters [11]. Particularly, Lewis acidic ILs (LAILs) such as [BMIm]TrCl₄ (BMIm = 1-n-butyl-3-methylimidazolium; Tr = Al, Ga, In) have emerged as versatile media for the low-temperature synthesis of homoatomic Bi cluster compounds including [Bi₅][AlCl₄]₃, [Bi₅][GaCl₄]₃, Bi₈[AlCl₄]₂, and Bi₈[GaCl₄]₂, to name a few [12,13,14]. Heterometallic Bi clusters in which electron-rich transition metals serve as guests within the Bi cluster host have likewise been discovered by introducing transition metal chloride salts into LAILs [15]. A few notable examples are [CuBi₈][AlBr₄]₂[Al₂Br₇] [15], [M@Bi₁₀][AlBr₄]₄ [16], [M@Bi₁₀][AlBr₄]₂[Al₂Br₇]₂ (M = Pd, Pt) [16], [Pd@Bi₁₀][Bi₂Sn₆Cl₂₂] [16], [Ni₂Bi₁₂][AlCl₄]₃[Al₂Cl₇] [17], [Rh₂Bi₁₂][AlBr₄]₄ [17], [Ru₂Bi₁₄Br₂][AlCl₄]₄ [18], AuBi_13.6Sn_2.4Cl_20.6 [19], and [Bi₈]Tl[AlCl₄]₃ [20], where “@” denotes the encapsulation of a transition metal within the cluster polycation, such as [Bi₁₀]⁴⁺. The broad applicability of ILs in inorganic cluster synthesis is further exemplified by the recent discovery of numerous non-Bi heterometallic cluster compounds, including [Mo₂Te₁₂]I₆ [21], Te₆[WOCl₄]₂ [21], and [Sb₇Se₈Br₂][AlX₄]₃·NbCl₅ (X = Cl, Br) [22], to name a few.

Particularly interesting is the family of compounds containing [Bi₁₀]⁴⁺ polycation, which can serve as a host for electron-rich heavy metals, such as Pt, Pd, and Au [15,16,23,24,25,26], or as a linking unit in more complex molecules, such as [Au₂Bi₁₀](PnBi₃Br₉)₂ (Pn = As, Sb, Bi) [27]. Three pentagonal antiprismatic intermetalloid clusters, [M@Bi₁₀]ⁿ⁺ (M = Pt, Pd, Au; n= +4/+5), were synthesized in LAILs at near-room temperature [15,16]. These clusters obey the Wade–Mingos electron-counting rules and are classified as arachno-type clusters [28,29]. In all cases, these cluster cations are charge-balanced by [AlX₄]⁻ (X = Cl, Br) tetrahedral anions.

In this paper, we report the one-pot synthesis, comprehensive structural characterization, and details of the electronic structure of the novel [Pd@Bi₁₀][AlCl₄]₄ phase, extending the family of known cluster compounds containing the [Bi₁₀]⁴⁺ structural unit. Although this compound has been previously identified [15], its structural characterization could not be completed due to poor crystal quality. However, through the use of high-quality single crystals and an extended exposure time, we have successfully solved the crystal structure of [Pd@Bi₁₀][AlCl₄]₄ and performed electronic structure calculations, providing insightful details regarding chemical bonding within the heterometallic Bi-bearing clusters.

2. Results and Discussion

2.1. Crystal Structure

The family of compounds containing [M@Bi₁₀]⁴⁺ clusters is compositionally and structurally abundant, vide supra [16,19]. Such structural diversity is particularly pronounced within isoelectronic [M@Bi₁₀][AlX₄]₄ (M = Pd, Pt; X = Cl, Br) compositions relevant to the title compound. For instance, [Pd@Bi₁₀][AlBr₄]₄ crystallizes in the tetragonal space group P4₂/n (ICSD 431980), whereas the Pt-bearing analog [Pt@Bi₁₀][AlBr₄]₄ exists in two polymorphic modifications, the high-temperature form P4₂/n (ICSD 431978) and the low-temperature form P$\overline{4}$ (ICSD 431977) [16]. In contrast, the chloride analog [Pt@Bi₁₀][AlCl₄]₄ exhibits lower symmetry and crystallizes in the triclinic space group P$\overline{1}$ (ICSD 124212) [15]. Similarly, the title [Pd@Bi₁₀][AlCl₄]₄ phase is isostructural to the Pt-containing analog and adopts the same triclinic archetype (

Table 1. Selected crystallographic data collection details and structure refinement details for [PdBi₁₀][AlCl₄]₄.

Chemical Formula	[PdBi10][AlCl4]4
fw/g mol−1	2871.32
Space group	$P\overline{1}$
a/Å	11.0233(5)
b/Å	26.1892(14)
c/Å	26.2687(14)
a/°	90.842(2)
β/°	92.1940(10)
γ/°	91.164(2)
V/Å3	7575.5(7)
Z	8
ρcalc./g cm–3	5.035
Radiation	AgKα (λ = 0.56086)
μ(Ag-Kα)/mm–1	25.978
2θ range for data collection/°	3.854 to 41.124
Index ranges	−13 ≤ h ≤ 13, −32 ≤ k ≤ 32, −32 ≤ l ≤ 32
Reflections collected/Independent reflections	463,841/ 31,100
R1 (I > 2σ(I)) [a]	0.0360
wR2 (I > 2σ(I)) [a]	0.0694
R1 (all data) [a]	0.0428
wR2 (all data) [a]	0.0712
Δρmax,min/e–·Å–3	2.848–(–2.020)
CCDC code	2448552

^[a] R₁ = Σ||F_o|–|F_c||/Σ|F_o|; wR₂ = [Σ[w(F_o²–F_c²)²]/Σw(F_o²)²]]^1/2, where w = 1/[σ2F_o² + (0.0046·P)² + (290.2168P)], and P = (F_o² + 2F_c²)/3.

, Table S1 and Table S2).

The crystal structure of the monoclinic [M@Bi₁₀][AlX₄]₄ polymorph is well established [15]; therefore, we provide only a brief description. The unit cell comprises four crystallographically unique [Pd@Bi₁₀]⁴⁺ polycations charge-balanced by 16 [AlCl₄]⁻ anions (Figure 1a). A typical polycation bears a pentagonal antiprismatic structure featuring a [Bi₁₀]⁴⁺ cage that encapsulates an endohedral Pd atom, similar in nature to the previously reported [Pt@Bi₁₀][AlX₄]₄ [16,19]. According to the Wade–Mingos rules, the [Bi₁₀]⁴⁺ shell is classified as an arachno-cluster with 26 skeletal electrons (N.B., only 6p electrons of Bi are considered to participate in skeletal bonding [28,29]). This corresponds to 13 electron pairs, which satisfies the n + 3 rule (where n = 10 cluster vertices), consistent with electron requirements for an arachno-type cluster.

The four crystallographically unique [Pd@Bi₁₀]⁴⁺ polycations are labeled as Pd1, Pd2, Pd3, and Pd4 in Figure 1b. Each formally neutral Pd atom exhibits tenfold coordination by surrounding Bi atoms, which is viewed as the maximum attainable CN in this framework, and is a hallmark of [M@Bi₁₀]ⁿ⁺-type cationic cluster compounds [15,16,19]. Within each cluster, every Bi atom is coordinated to four neighboring Bi atoms (two within the pentagonal face and two within the triangular face) and one Pd atom, resulting in a fivefold coordination environment. The pentagonal bases of the [Bi₁₀]⁴⁺ antiprisms are parallel to each other, with an approximate distance of 2.55 Å between planes.

The symmetric nature and structural identity of the four crystallographically unique clusters are evident from the examination of bond lengths and bond angles (Figure 2, Table S2). The Pd–Bi bond lengths are in the range of 2.9636(10)–3.0152(11) Å for Pd1–Bi, 2.9573(11)–3.0083(11) Å for Pd2–Bi, 2.9364(10) –3.0098(10)Å for Pd3–Bi, and 2.9449(11)–3.0236(10) Å for Pd4–Bi, consistent with those of previously reported [M@Bi₁₀]⁴⁺ (M = Pt, Pd) clusters [15,16], though slightly longer than the sum of the covalent radii of Pd and Bi. The Bi–Bi bond lengths at the base of the cluster span from 3.1168(7) to 3.1964(7) Å, with the largest distribution for the Pd1-based cluster (Table S2) and are in good agreement with those observed in related [M@Bi₁₀]ⁿ⁺ clusters and in the [Bi₈]²⁺ fragment [13,14,15,16]. The triangular faces of the [Bi₁₀]⁴⁺ unit exhibit slightly shorter average Bi–Bi distances, ranging from 3.0656(9) to 3.1552(9) Å, with the largest distribution for the Pd4-based cluster, although these observations are in good agreement with previous reports, the results of our calculations (vide infra), and the expected interatomic distances for homoatomic Bi–Bi contacts in Zintl bismuthides [30,31]. The Bi–Bi–Bi bond angles within the pentagonal base vary from 105.08(2)° to 110.87(2)°, with the largest distribution for the Pd4-based cluster, closely approximating the ideal pentagonal angle of 108° (Table S2). Please note that the angles for the other three clusters are within the same range, but less widely distributed, being closer to 108° (Table S2). The respective angles within the triangular face are close to 60°, as expected for an equilateral triangle (Table S2). It is important to note that the four clusters in the isostructural [PtBi₁₀][AlCl₄]₄ phase are also symmetrically identical, but possess minimal discrepancy within the Pt–Bi and Bi–Bi bonding, comparable to the discussed title compound [15].

The Al–Cl bond lengths fall between approximately 2.115 and 2.156 Å, within the expected range for Al–Cl bonding and comparable to those in reported Bi-bearing clusters with the [AlCl₄]ˉ counter anion [12,14]. The observed Cl–Al–Cl bond angles show only minimal deviation from the ideal tetrahedral angle of 109.4°, consistent with sp³ hybridization of the Al atoms, as further discussed in the following section.

2.2. Electronic Structure

Following single-crystal XRD analysis, DFT calculations were performed for the [Pd@Bi₁₀]⁴⁺ polycation using various exchange-correlation functionals within the TURBOMOLE program. Geometry-optimized interatomic distances and bond angles were compared with the experimental values (

Table 2. Selected interatomic distances of the optimized equilibrium geometry of the [Pd@Bi₁₀]⁴⁺ polycation, as obtained in the def2-TZVP basis using different DFT functionals.

Bond Distance (Å)/Angle (°)	TPSSh	TPSSh with COSMO	M06-2X	B3-LYP	Experimental
d (Bi–Pd)	3.0196–3.0212	2.9720–2.9828	3.0212–3.0228	3.0717–3.0736	2.9364(10)–3.0236(10)
d (Bi–Bi)Pentagon	3.2036–3.2042	3.1434–3.1464	3.2117–3.2158	3.2645–3.2672	3.1168(7)–3.1964(7)
d (Bi–Bi)Triangle	3.1001–3.1005	3.0785–3.0791	3.0798–3.0811	3.1359–3.1374	3.0656(9)–3.1552(9)
∠(Bi–Bi–Bi)Pentagon	107.95–108.03	107.98–108.02	107.96–108.01	107.94–108.05	105.08(2)–110.87(2)
∠(Bi–Pd–Bi)	61.75–61.78	63.84–63.88	61.30–64.22	61.36–61.40	62.05(2)–64.61(3)
∠(Bi–Bi–Bi)Triangle	58.88–62.23	59.27–61.46	58.55–62.87	58.61–62.76	57.812(16)–62.393(16)

) to validate the accuracy of the calculated structural model. The computational results showed good agreement with the experimental data across all the tested functionals. Notably, the closest correspondence was observed for calculations employing the COSMO solvation method with TPSSh functional.

The Electron Localization Function (ELF) analysis of the geometry-optimized [PdBi₁₀]⁴⁺ cluster reveals a uniform degree of electron localization across the bases and faces of the cluster. The ELF isosurface maps (Figure 3a) further highlight distinct bonding interactions among the Bi atoms with clear localized domains between atomic pairs, illustrating the symmetric nature of the cluster [32,33]. On the other hand, we did not observe significant localized domains between Pd and Bi atoms, which corroborates the previously observed host–guest nature of the bonding within the [M@Bi₁₀]⁴⁺ polycation. The HOMO–LUMO energy gaps for the cluster, computed using different functionals, are 2.31 eV (TPSSh), 4.31 eV (M06-2X), and 2.69 eV (B3-LYP), indicating functional-dependent variation in the predicted electronic properties. However, all the values are relatively large, implying the stability of the structural fragment.

To better understand the interactions within the [PdBi₁₀]⁴⁺ cluster, Natural Population Analysis (NPA) was performed via Natural Bond Orbital (NBO) calculations using the TURBOMOLE program, based on the geometry optimized with the TPSSh functional using the COSMO solvation method. The effective charges were calculated for all the atoms in the cluster (Table S3 and S4). The more electronegative Pd atom (EN = 2.28) carries a partial negative charge of ca. –0.24, while all the Bi atoms (EN = 2.02) exhibit positive charges of ca. +0.42 (Figure 3a). These results corroborate well with the notation that the negatively charged Pd center stabilizes the cluster through electron donation, whereas the positively charged Bi atoms reflect a partial depletion of electron density. The charge distribution is consistent with previous reports in which the NPA charges of Pd and Bi were calculated as −0.5 and +0.45, respectively [19]. Such charge separation is indicative of a polarized bonding framework centered around the endohedral Pd, as opposed to a fully delocalized metallic bonding environment, which indicates the covalent nature within the cluster framework.

However, the nature of the bonding between Pd and Bi within the [PdBi₁₀]⁴⁺ cluster remains debatable. Early reports suggested a host–guest scenario with negligible bonding interaction between the encapsulated metal and the Bi₁₀ framework [16,19], which was also observed in the ELF analysis described above. On the other hand, more recent ELF analyses of multiple clusters have indicated non-negligible Pd–Bi bonding interactions [34].

To further investigate the electronic structure of the [Pd@Bi₁₀][AlCl₄]₄ phase and assess orbital overlap, we performed density of states (DOS) and crystal orbital Hamilton population (COHP) calculations using the TB-LMTO-ASA program. The results are presented in Figure 4. Major contributions to the valence band maxima (VBM) are provided by the Pd d, Bi p, and, to a lesser extent, Cl p orbitals. Notably, there is a clear overlap between the Pd d and Bi p states in the region just above –2.2 eV, which may be indicative of Pd–Bi interaction within the polycation. These contacts show a combination of bonding (between −2 eV and −0.9 eV) and antibonding (−0.9 eV to VBM) states below the Fermi level (Figure 4f). Pd–Bi bonding is quantified with the calculated −ICOHP values of ca. 0.92 eV/bond, which is indicative of a weak interaction, as opposed to the calculated −ICOHPs of ca. 2.43 eV/bond for strong covalent Nb–As bonding within the [NbAs₁₀]⁵⁻ cluster [33].

The overall dominance of the Bi 6p states in the valence band is consistent with the Bi-rich composition of the title compound. Interestingly, the minimal contribution of the Bi 6s orbitals supports the presence of localized lone pairs and suggests poor s–p orbital overlap. In contrast, the Al s, p, and d orbitals overlap in the same energy window below −2 eV; thus, averaging their energies suggests a perfect symmetry with the nearly ideal tetrahedral geometry of the [AlCl₄]⁻ anions. This is further supported by the significant contribution of the Cl p states in the same energy range. The conduction band minima (CBM) is predominantly derived from the Bi p states.

Before we discuss the details of Bi–Bi interactions within the cluster, we want to reiterate that these bonds can be classified within pentagonal bases and triangular faces (Figure 2). Although all of these bonds are optimized at the Fermi level (Figure 4f), negligible antibonding domains are observed for Bi–Bi interactions within the pentagon down to −0.4 eV. In combination with the slightly longer bond distances, compared to Bi–Bi bonds within the triangular faces (Table S2), this may be an indication of slightly weaker covalent interactions. Indeed, the average −ICOHP values for (Bi–Bi)_triangle and (Bi–Bi)_pentagon are –3.8 eV/bond and –2.25 eV/bond, respectively, which confirms a more pronounced covalent nature of the former.

Electronic structure calculations reveal that [Pd@Bi₁₀][AlCl₄]₄ exhibits semiconducting behavior with a calculated bandgap of 0.71 eV, which is consistent with the structurally related cluster compound [Pd@Bi₁₀][Bi₂Sn₆Cl₂₂], which features a comparable bandgap of 0.9 eV and similarly displays a black appearance [16,25].

3. Materials and Methods

3.1. Synthesis

Commercially available starting materials, anhydrous PdCl₂ (Pressure Chemicals Co., Pittsburgh, PA, USA, 60% Pd)) and elemental Bi (Sigma-Aldrich, 99.99%), were used as received. AlCl₃ (Thermo Scientific, Waltham, MA, USA, 99%) was dried at 35 °C for 48 h, and [BMIm]Cl (Alfa Aesar, Waltham, MA, USA, 96%) was vacuum dried at 130 °C for 48 h before use. Anhydrous BiCl₃ (Thermo Scientific, Waltham, MA, USA, 99.9%) was sublimated prior to use.

All synthetic and post-synthetic manipulations were carried out in an argon-filled glovebox with O₂ level and H₂O level less than 1 ppm. The synthesis was performed in a vacuum-sealed quartz ampule with a diameter of 14 mm and a length of 100mm. PdCl₂ (14.2 mg), Bi (153.3 mg), AlCl₃ (450 mg), [BMIm]Cl (150 mg), and BiCl₃ (21.0 mg) were added to the ampule, wherein the IL mixture began to liquefy immediately, yielding a light-brown solution. The sealed ampule was placed in a slightly tilted tube furnace to ensure that all the reaction materials settled at the bottom. The reaction mixture was maintained at 180 °C for 48 h and then cooled to room temperature at the rate of 6 °C/h, consistent with a previous report [16]. Upon completion, black, needle-shaped crystals of [Pd@Bi₁₀][AlCl₄]₄ were obtained. These crystals were found to be air-sensitive, which complicated further characterization of their properties. As a minor byproduct, red-colored, cube-shaped crystals of Bi₅[AlCl₄]₃ were identified [12]. The two phases were easily distinguished under a microscope.

3.2. Single Crystal X-Ray Diffraction

The crystal structure of [Pd@Bi₁₀][AlCl₄]₄ was investigated using the Single Crystal X-ray Diffraction (SCXRD) method. An Ag Kα (λ = 0.56086 Å) radiation source was used to capture data on a Bruker D8 Venture DUO, with a Photon III C14 detector diffractometer, under a cold nitrogen stream at 100(2) K. Suitable single crystals were chosen, sliced to the required dimensions of under 100 μm, and mounted on MiTeGen plastic loops. Data integration and absorption correction were performed using the SAINT and SADABS programs (v.8.40B), respectively, both implemented with the APEX4 software suite (v2021.4.0) [35,36]. A multi-scan absorption correction was applied, yielding minimum and maximum transmission factors of 0.1447 and 0.2535, respectively. The crystal structure was solved using the SHELXT program (v.2018/2) and refined with the SHELXL program (v.2018/1), using Olex2-1.5 as a graphical interface [37,38,39]. An isotropic extinction correction was applied during the refinement, yielding a refined extinction coefficient of 0.000036(2). Atomic coordinates and labels were standardized using the STRUCTURE TIDY program [40]. Structural illustrations were prepared using the CrystalMaker software (v.11.0.1) [41].

3.3. Electronic Structure Calculations

Electronic structure calculations were performed using two complementary approaches. First, molecular quantum chemical calculations were performed on the isolated [Pd@Bi₁₀]⁴⁺ polycation using the TURBOMOLE program v.7.8 [42]. Density Functional Theory (DFT) computations were performed with TPSSh, M06-2X, and B3-LYP functionals including Effective Core Potential (def2-ECP) with def2-TZVP basis [43,44,45,46,47]. The COSMO solvation method was used in the TPSSh functional with relative permittivity ε = 100 [48]. Geometry optimizations were performed with all the functionals with tight thresholds for the energy and gradient (scfconv = 7) [49]. Electron Localization Function (ELF) analysis and Natural Population Analysis (NBO) were conducted at the TPSSh level with the COSMO solvation method [50,51]. Molecular visualizations were generated using the TmoleX 2024 package [52].

Second, density of states (DOS) calculations for the [Pd@Bi₁₀][AlCl₄]₄ structure were performed at the DFT level using the TB-LMTO-ASA code [53]. The standardized experimental unit cell parameters (Table 1) and atomic coordinates (Table S1) were used without structure optimization. The atomic sphere approximation (ASA) was satisfied by introducing empty spheres, and the von Barth–Hedin functional was employed [54]. After self-consistent convergence, a 17 × 7 × 7 k-point mesh was employed to sample the Brillouin zone. The Fermi level was set to E_F = 0 eV. A basic set of orbitals included Pd [5s, 5p, 4d, and (4f)], Bi [6s, 6p, (6d), (5f)], Al [3s, 3p, (3d)] Cl [(4s), 3p, (3d)], with the downfolded orbitals treated with the Löwdin downfolding method given in parentheses. Chemical bonding was studied using crystal orbital Hamilton population (COHP) curves using the module within the TB-LMTO-ASA code [55].

4. Conclusions

In this work, we successfully reproduced and structurally characterized a novel intermetallic Bi cluster compound, [Pd@Bi₁₀][AlCl₄]₄, via a one-pot reaction using the LAIL [BMIm]·4.2[AlCl₄]₄. This charge-balanced compound comprises 0D pentagonal antiprismatic [Pd@Bi₁₀]⁴⁺ polycations stabilized by isolated [AlCl₄]⁻ tetrahedral anions. The symmetric nature of the cluster was confirmed through detailed structural analysis and further validated by DFT calculations. Analysis of the electronic structure revealed non-negligible Pd–Bi bonding interactions, although consistent with the host–guest model, and a charge distribution consistent with electronegativity trends. Further electronic structure calculations performed with the TB-LMTO-ASA code indicated semiconducting behavior with a bandgap of 0.71 eV. This work broadens the scope of [M@Bi₁₀]⁴⁺ cluster chemistry and supports prior findings on the viability of LAILs in stabilizing complex intermetalloid clusters under mild conditions, while also providing new insights into covalent transition metal–pnictogen interactions.