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Despite its electron deficiency, boron can form multiple bonds with a variety of elements. However, multiple bonds between boron and main-group metal elements are relatively rare. Here we report the observation of boron-lead multiple bonds in PbB₂O^–and PbB₃O₂^–, which are produced and characterized in a cluster beam. PbB₂O^–is found to have an open-shell linear structure, in which the bond order of B☱Pb is 2.5, while the closed-shell [Pb≡B–B≡O]^2–contains a B≡Pb triple bond. PbB₃O₂^–is shown to have a Y-shaped structure with a terminal B = Pb double bond coordinated by two boronyl ligands. Comparison between [Pb≡B–B≡O]^2–/[Pb=B(B≡O)₂]^–and the isoelectronic [Pb≡B–C≡O]^–/[Pb=B(C≡O)₂]⁺carbonyl counterparts further reveals transition-metal-like behaviors for the central B atoms. Additional theoretical studies show that Ge and Sn can form similar boron species as Pb, suggesting the possibilities to synthesize new compounds containing multiple boron bonds with heavy group-14 elements.

Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB₈⁻, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB₈⁻species changes fromC_stoC_7vsymmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All theC_7v-LnB₈⁻clusters can be viewed as a monovalent Ln(I) coordinated by a η⁸-B₈²⁻doubly aromatic ligand. The B₇³⁻, B₈²⁻, and B₉⁻series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C₅H₅⁻, C₆H₆, and C₇H₇⁺, respectively, with similar trends of size and charge state and they are named collectively as “borozenes”. Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes.

Joint photoelectron spectroscopy and first-principles theory investigations indicate that the Pb-doped PbB₂(BO)_n⁻clusters (n= 0−2) undergo a transformation from σ + π doubly aromatic triangle PbB₂⁻to PbB₄(BO)₂^−/0complexes with a B≡B triple bond.

We report the experimental observation and spectroscopic characterization, and structure and bonding analyses of copper–borozene complexes.

Metal-boron triple bonds are rare due to the electron deficiency of boron.

We present an investigation on the structures and chemical bonding of two Bi-doped boron clusters BiBn− (n = 4, 5) using photoelectron spectroscopy and theoretical calculations. The electron affinities of BiB4 and BiB5 are measured to be 2.22(2) eV and 2.61(2) eV, respectively. Well-resolved photoelectron spectra are obtained and used to compare with theoretical calculations to verify the structures of BiB4− and BiB5−. Both clusters adopt planar structures with the Bi atom bonded to the periphery of the planar Bn moiety. Chemical bonding analyses reveal that the Bn moiety maintains σ and π double-aromaticity. The Bi atom is found to induce relatively small structural changes to the Bn moiety, very different from transition metal-doped boron clusters.

We report a study on the electronic structure and chemical bonding of the BiB molecule using high-resolution photoelectron imaging of cryogenically cooled BiB− anion. By eliminating all the vibrational hot bands, we can resolve the complicated detachment transitions due to the open-shell nature of BiB and the strong spin–orbit coupling. The electron affinity of BiB is measured to be 2.010(1) eV. The ground state of BiB− is determined to be 2Π(3/2) with a σ2π3 valence electron configuration, while the ground state of BiB is found to be 3Σ−(0+) with a σ2π2 electron configuration. Eight low-lying spin–orbit excited states [3Σ−(1), 1Δ(2), 1Σ+(0+), 3Π(2), 3Π(1), 1Π(1)], including two forbidden transitions, [3Π(0−) and 3Π(0+)], are observed for BiB as a result of electron detachment from the σ and π orbitals of BiB−. The angular distribution information from the photoelectron imaging is found to be critical to distinguish detachment transitions from the σ or π orbital for the spectral assignment. This study provides a wealth of information about the low-lying electronic states and spin–orbit coupling of BiB, demonstrating the importance of cryogenic cooling for obtaining well-resolved photoelectron spectra for size-selected clusters produced from a laser vaporization cluster source.