Synthesis and reactions of (tert-butylimido)bis(η-cyclopentadienyl)niobium cations: NMR evidence for d0 olefin cations [Nb{(η-C5H5)2}(NtBu)(η-C2H4)][B(C6F5)4] and [Nb{(η-C5H4)CMe2(η-C5H4)}(NtBu)(η-C2H3Me)][B(C6F5)4]
Synthesis and reactions of (tert-butylimido)bis(η-cyclopentadienyl)niobium cations: NMR evidence for d0 olefin cations [Nb{(η-C5H5)2}(NtBu)(η-C2H4)][B(C6F5)4] and [Nb{(η-C5H4)CMe2(η-C5H4)}(NtBu)(η-C2H3Me)][B(C6F5)4]