The Front Cover shows the art gallery within the Beryllium Centre, which exhibits some of the latest samples of beryllium art. The current exhibition features the solid state structure of dinuclear [(PPh₃)BeCl₂]₂. In this piece, only one phosphine ligand can be accommodated due to the steric bulk of PPh₃. However, two smaller ligands, like PMePh₂, can find a place in the Beryllium Centre. Therefore, size restrictions apply with a threshold cone angle from 136° to 145°. This parameter dictates whether single or double admission is allowed. Furthermore, the electron donating abilities of the ligands determine whether they behave as spectator ligands or attack the solvent. Therefore, the walls are covered with advertising flyers by the stronger donors. More information can be found in the Research Article by M. R. Buchner and S. I. Ivlev.

The threshold cone angle of phosphines was determined, below which two ligands can be accommodated in the first ligand sphere of beryllium dihalide fragments. While [(PMe₂Ph)₂BeX ₂] (X=Cl, Br, I) and [(PMePh₂)₂BeX ₂] are mononuclear complexes, [(PPh₃)BeX ₂]₂ is dinuclear and exhibits dynamic behaviour in solution due to phosphine dissociation. This high dynamicity is the reason for halide exchange with dichloromethane.

Abstract

The phosphine complexes of beryllium chloride, bromide and iodide, [(PMe₂Ph)₂BeX ₂], [(PMePh₂)₂BeX ₂] and [(PPh₃)BeX ₂]₂ (X=Cl, Br, I) were prepared and characterised with multinuclear NMR spectroscopy. Additionally the molecular structure of dinuclear [(PPh₃)BeCl₂]₂ was determined with single crystal X-ray diffraction techniques. The threshold cone angle of the phosphines, below which two ligands can coordinate to the beryllium dihalide fragments, is between 136° and 145°. Halide-chloride exchange in dichloromethane is observed for [(PPh₃)BeBr₂]₂ and [(PPh₃)BeI₂]₂, which leads to the formation of [(PPh₃)BeCl₂]₂. Due to the relatively low Lewis basicity of PPh₃, it almost exclusively acts as a spectator ligand with only little formation of phosphonium cations.