Angew.Chem.Int.Ed.http://doi.org/dgj5xp(2012)

Credit: © 2012 WILEY

The 'classical' shape of four-coordinate nitrogen, carbon and boron compounds is tetrahedral, as predicted by the valence-shell electron-pair repulsion theory that all chemists learn at an early stage. Organic chemists can use a variety of thermodynamic and kinetic methods to force carbon into unusual geometries, but inorganic chemists have a powerful trick up their sleeve: metals. Electron-rich transition metals can form π-backbonds to nitrogen, carbon or boron, which can stabilize non-tetrahedral geometries. Several examples of square-planar carbon compounds exist, but boron has so far only been forced into three-coordinate T-shaped planar geometries.

Now, Holger Braunschweig and colleagues from the University of Würzburg have made a compound where a four-coordinate boron centre is roughly planar. Starting from a linear iron–boron–chromium compound, they added two platinum phosphine compounds that formed the other two points of the square. The linear core is retained in the resulting compound and the two platinum groups are slightly bent out of the plane. The team also made a roughly square-planar boron compound with a linear manganese–boron–manganese core, with platinum and gold units also present.

Braunschweig and colleagues considered the bonding using simple valence-bonding ideas and decided a neutral boron atom datively bonding to a neutral chromium atom was a better description than a dianionic borylene ligand, because the former obeys the octet rule. Most remarkably, with no stiff bridging ligands, neither compound is considered to be under strain.