Synthesis, crystal and molecular structure of [{C5Me5FeC5Me4CH2}+B{C6H3(CF3)2}4−], the first example of a structurally characterized primary ferrocenylcarbocation

The synthesis and the first X-ray structural study of a salt of the primary ferrocenylcarbocation [{C5Me5MC5Me4CH2}+B{C6H3(CF3)2}4−] (M=Fe, 1a) are reported. The CCp–Cα bond shows considerable inclination relative to the plane of the cyclopentadienyl ring so that the Cα atom becomes closer to the metal centre, the inclination angle α being equal to 23.6°. The Fe–Cα distance (2.567(12) Å) is still significantly longer than the covalent Fe–C σ-bond, which may be considered as an indication that the donor–acceptor interaction of the carbocationic centre with the metal electron pair is substantially weaker than in the earlier studied Ru- and Os-analogues (1b, 1c) wherein the positive charge was mostly localized on the metal centre (metallonium cations). Thus, complex 1a obviously preserves much more of the carbocationic state than its heavier analogues.