This is understandable because main-group metal elements have valence ns and np orbitals with large differences in orbital radii, decreasing the hybridization of these orbitals and making it difficult for heavy elements to form strong multiple bonds. In fact, complexes with transition-metal-boron bond lengths shorter than B≡M triple bonds 28, 29 and even B ≣M quadruple bonds 30, 31 have been characterized by joint gas-phase experimental and ab initio theoretical studies.Ĭompared with the transition-metal-boron multiple-bond complexes, compounds with multiple bonding between boron and main-group metal elements are rare 24, even though multiple bonds of boron with light main-group elements are common.
The first electron-precise transition-metal-boron triple-bond complex was identified recently by combined photoelectron spectroscopy (PES) and quantum chemistry calculations, in the linear ReB 2O – species with a B≡Re triple bond 27. Several transition-metal complexes with B–M bond lengths shorter than B=M double bonds have been characterized in both solid compounds and gaseous molecules 15, 16, 17, 18, 19, among which some have B–M bond lengths comparable to those computed from Pyykkö’s triple-bond covalent radii. Boron-metal triple-bond characters were first suggested in with the B–Cr bond length of 1.871 Å 25, slightly shorter than the B=Cr double-bond length of 1.89 Å derived from Pyykkö’s covalent radii 26. Since the syntheses of the first transition-metal borylene complexes 12, 13, considerable progresses have been achieved in this area 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24. However, the B–M bond lengths vary in a wide range, depending on the ligands and the R group 11. The similarities between borylenes and carbenes (:CR 2) suggest the bonding between boron and metal should be a double bond. The bonding between the metal and borylene fragment is interpreted as B→M σ-donation and M(dπ)→B back-donation 7, 8, 9, 10. Boron is also capable of forming multiple chemical bonds with transition metals, such as in borylene (:BR) compounds, which usually involve a transition metal (M) with different ligands (L n), L nMBR 6. 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.ĭue to its electron deficiency, boron tends to form multicenter bonds in both its compounds and at nanoscales 1, 2, 3, 4, 5. Comparison between 2–/ – and the isoelectronic –/ + carbonyl counterparts further reveals transition-metal-like behaviors for the central B atoms. PbB 3O 2 – is shown to have a Y-shaped structure with a terminal B = Pb double bond coordinated by two boronyl ligands. PbB 2O – is found to have an open-shell linear structure, in which the bond order of B☱Pb is 2.5, while the closed-shell 2– contains a B≡Pb triple bond. Here we report the observation of boron-lead multiple bonds in PbB 2O – and PbB 3O 2 –, which are produced and characterized in a cluster beam. However, multiple bonds between boron and main-group metal elements are relatively rare. Despite its electron deficiency, boron can form multiple bonds with a variety of elements.
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