[(Cp(2)M)(2)B(9)H(11)] (M = Zr or Hf): early transition metal ‘guarded’ heptaborane with strong covalent and electrostatic bonding

Among the series of stable closo-borate dianions, [B(n)H(n)](2–), the X-ray crystallographic structure of [B(7)H(7)](2–) 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, [(Cp(2)M)(2)B(9)H(11)] (Cp = η(5)-C(5)H(5); M = Zr or Hf). The structures of [(Cp(2)M)(2)B(9)H(11)] contain a pentagonal bipyramidal B(7) core, coordinated by two {Cp(2)M} and two {BH(2)} units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp(2)M)(2)B(9)H(11)] complexes are substantially more stable than the parent dianion, in either [B(7)H(7)](2–) or ((n)Bu(4)N)(2)[B(7)H(7)]. 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 exo-{Cp(2)M} units, as well as electrostatic interactions between the {Cp(2)M} units and the B(7) core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.