Metallacarboranes Incorporating an <i>arachno</i>-η⁶-C₂B₉ Ligand

Reduction of [Li(DME)3][{η5:η5-Me2C(C5H4)(C2B9H10)}MCl2] (M = Zr (1), Hf (2)) by an excess amount of Na metal gave the mixed-valent Zr(III)/Zr(IV) complex {Na3(THF)8}{[η5:η6-Me2C(C5H4)(C2B9H10)]Zr}2 (3) and high-valent Hf(IV) complex [Li(THF)4]2[{η5:η6-Me2C(C5H4)(C2B9H10)}Hf]2 (4), respectively. Treatment of 4 with [PPN]Cl (PPN = bis(triphenylphosphine)iminium cation) afforded the cation exchange product [PPN]2[{η5:η6-Me2C(C5H4)(C2B9H10)}Hf]2 (5). It was suggested that the nido η5-C2B9H10R2− unit accepted formally two electrons from Na during the reduction process to form an arachno-η6-C2B9H10R4− fragment via the cage C−C bond cleavage. Molecular structures of 3 and 5 were confirmed by single-crystal X-ray analyses.