Unexpected Continuity in Bonding Trends Across Period 2 Elements

Due to its toxicity, the chemistry of beryllium is poorly understood. Since the lightest elements form the basis for fundamental models of chemical bonding, there is a need for better insight into the properties of beryllium, and here the chemistry of the homo-elemental Be–Be bond is of particular interest.

Josef T. Boronski, Simon Aldridge and colleagues, University of Oxford, UK, have investigated the ligand metathesis chemistry of diberyllocene (CpBeBeCp, 1), a stable complex with a Be–Be bond. They have synthesized two new complexes with Be–Be bonds: Cp*BeBeCp (2) and [K{(HCDippN)₂BO}₂]BeBeCp (3; Dipp = 2,6-diisopropylphenyl). Ligand metathesis at a [Mg–Mg]²⁺ unit has not been reported before. The ability of complex 1 to undergo ligand metathesis while retaining the Be–Be bond, supports the chemical similarities between [Zn–Zn]²⁺ and [Be–Be]²⁺ units.

The complexes 2 and 3 are both unsymmetrical and thus have Be–Be bonds that are polarized, although to very different degrees. Quantum chemical calculations suggest that 3 can be formulated as a mixed-valence Be⁰/Be^II complex, which is supported by experimental evidence such as single crystal X-ray diffraction (SCXRD) measurements and reactivity studies.

In particular, complex 3 can transfer a beryllyl anion ([BeCp]^–) to an electrophilic organic substrate. The reaction of complex 3 with [CPh₃][B(C₆F₅)₄] produces previously unreported complexes Be(NHBO)₂ and CpBe(CPh₃), as well as K[B(C₆F₅)₄], through the transfer of the nucleophilic [CpBe]⁻ anion. This reactivity can be compared to that of sp²-sp³ diboranes. Despite the respective metallic and non-metallic nature of these elements, the reactivities of the homo-elemental bonds of beryllium and boron have parallels, and according to the researchers, this work shows that the bonding trends over period 2 are more continuous than may have been previously appreciated.