A joint research team led by Prof. Sun Zhongming from Nankai University and Prof. Zhao Lili from Nanjing Tech University has achieved a breakthrough in transition metal chemistry. Their research outcome, titled “Synthesis of Triple-Decker Sandwich Compounds Featuring a M–M Bond Through Cyclo-Bi₅ and Cyclo-Sb₅ Rings”, was recently published in Nature Chemistry. Using Zintl-phase compounds as Sb/Bi sources, the researchers successfully synthesized Cyclo-Bi₅ and Cyclo-Sb₅ rings and achieved the first-ever isolation of Bi₅⁻ anions in a one-step reaction at room temperature. They conducted detailed experimental characterization and theoretical calculations, achieving a significant breakthrough in the field of main-group and transition metal coordination chemistry.

Ligands comprising π-aromatic pentagonal rings have been widely employed in transition metal chemistry. Since the discovery of ferrocene, the cyclopentadienyl anion (C₅H₅⁻), particularly, has been one of the most frequently used ligands. Pn₅⁻, a purely inorganic ligand, can be derived by substituting CR groups in the general formula (C₅R₅)⁻ (where R = H or organic substituents) with isoelectronic Group 15 elements (Pn). Bismuth (Bi), the heaviest pnictogen homolog, has remained elusive and represents a significant synthetic challenge in inorganic synthetic chemistry that persists to date. Lighter pnictogen homologs have been successfully synthesized as such ligands: N₅⁻ can be synthesized via C-N bond cleavage of aryl pentazides with m-CPBA and transition metal complexes serving as stabilizing agents (Nature 2017, 549, 78-81); P₅⁻ and As₅⁻ are typically synthesized through reduction of white phosphorus or yellow arsenic, or alternatively from organic cyclic phosphorus/arsenic compounds. These reactions require elevated temperatures and prolonged reaction times, ultimately yielding stable triple-decker sandwich compounds (Angew. Chem. Int. Ed. 1986, 25, 363-364; J. Am. Chem. Soc. 1982, 104, 4727-4729). The synthesis of Sb₅⁻ and Bi₅⁻ proves more challenging, as these heavy pnictogen atoms preferentially form clusters rather than multiply-bonded rings. To date, only one Sb₅⁻ species has been reported, synthesized from ^tBu₄Sb₄ as the starting material (Angew. Chem. Int. Ed. 2000, 39, 4148-4150) (Figure 1).

Figure 1. Reported triple-decker sandwich compounds and the synthetic route developed in this work

The research team led by Prof. Sun and Prof. Zhao used Zintl-phase compounds as Sb/Bi sources to synthesize cyclo-Sb₅ and cyclo-Bi₅ and achieve the first-ever isolated Bi₅^- via a one-step reaction at room temperature. The anionic complexes [CpV(cyclo-Sb₅)V-Cp]²⁻ and [CpNb(cyclo-Bi₅)Nb-Cp]²⁻ feature pentagonal bipyramidal core structures, where the planar Pn₅ rings are sandwiched between two transition metals (V or Nb). The unique V–V and Nb–Nb bonds penetrating the Sb₅/Bi₅ centers exert a significant impact on the electronic structure of these clusters and distinguish them from previously reported neutral sandwich compounds (Figure 2).

Figure 2. The structure and mass spectrum of the cluster [CpNb(cyclo-Bi₅)NbCp]²⁻ (Isostructural to [CpV(cyclo-Sb₅)VCp]²⁻)

Adaptive Natural Density Partitioning (AdNDP) analysis of [V₂Cp₂Sb₅]²⁻ reveals only two 3c-2e bonds in the Sb₅ ring, in addition to ten 2c-2e V-Sb bonds, five 1c-2e lone pairs, as well as a V-V bond with high occupancy up to 1.93. AdNDP analysis of [Nb₂Cp₂Bi₅]²⁻ reveals bonding patterns analogous to its [V₂Cp₂Sb₅]²⁻ counterpart (Figure 3). 

Figure 3. QTAIM and AdNDP analyses of [V₂Cp₂Sb₅]²⁻ and [Nb₂Cp₂Bi₅]²⁻

To verify the existence of a 2c-2e M–M bond penetrating the E₅ ring center in the M(cyclo-E₅)M core unit, the research team conducted EDA-NOCV calculations on [V₂Cp₂Sb₅]²⁻ and [Nb₂Cp₂Bi₅]²⁻. They used cyclo-Sb₅/Bi₅ and (VCp)₂/(NbCp)₂ as interacting fragments and conducted analyses under varying charge and electronic states. Analysis of the deformation densities and bonding orbitals reveals that ten orbital interactions originate from M-E₅ cage bond formation, while one distinct orbital interaction is unequivocally associated with M-M bonding, with negligible contribution (ΔEₒᵣ_b(4) from the E₅⁻ ring (∆Eₒᵣ_b(4) in [V₂Cp₂Sb₅]²⁻ and ∆Eₒᵣ_b(1) in [Nb₂Cp₂Bi₅]²⁻), which nevertheless represents the strongest orbital contribution in these molecules. The Nb–Nb bond formed through the orbital interaction ΔE_orb(1) primarily involves the HOMO-3 and LUMO of the [(NbCp)₂]⁻ fragment. The corresponding charge flow eigenvalue (0.074) confirms negligible contribution from the [cyclo-Bi₅]^- unit. This phenomenon is mirrored in the ∆E_orb(4) interaction of [V₂Cp₂Sb₅]^2- (Figure 4), conclusively demonstrating that both V–V and Nb–Nb bonds physically penetrate the central cavity of the E₅ rings.

Figure 4. The deformation density (Δρ) and corresponding interfragment orbital interaction associated with the dominant ∆Eorb(1) term in [Nb₂Cp₂Bi₅]^2-

Dr. Xu Yuhe from NKU and Yang Xing from NTU equally as co-first authors. Corresponding authors include Prof. Sun Zhongming from NKU, Prof. Zhao Lili from NTU, and Prof. Gernot Frenking from Philipps-Universität Marburg, Germany (Adjunct Professor at NTU).

Read the paper at https://www.nature.com/articles/s41557-025-01765-4

（Edited and translated by Nankai News Team.)