[(CpM)BH] (M = Zr or Hf): early transition metal 'guarded' heptaborane with strong covalent and electrostatic bonding.

Among the series of stable -borate dianions, [B H ], the X-ray crystallographic structure of [BH] was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(CpM)BH] (Cp = η-CH; M = Zr or Hf). The structures of [(CpM)BH] contain a pentagonal bipyramidal B core, coordinated by two {CpM} and two {BH} units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(CpM)BH] complexes are substantially more stable than the parent dianion, in either [BH] or ( BuN)[BH]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two -{CpM} units, as well as electrostatic interactions between the {CpM} units and the B core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.