Reaction of B(C6F5)3 with zirconium and hafnium benzyl diene complexes. The crystal and molecular structures of Cp″Zr(C6F5){η4-CH2CMeCHCHB(C6F5)2} and [Cp″Hf(2,3-Me2C4H4)(OEt2)][PhCH2B(C6F5)3] [Cp″ = 1,3-(SiMe3)2C5H3]