The cations [(RCp) 2M(–CC–CH 3)(THF) +] ( 2a– d) [(RCp) 2M=Cp 2Ti, Cp 2Zr, (MeCp) 2Zr, and Cp 2Hf] were generated in situ by treatment of the respective bis(propynyl) Group 4 metallocenes (RCp) 2M(–CC–CH 3) 2 ( 1a– d) with N, N-dimethylanilinium tetraphenylborate. Addition of excess tert-butylisocyanide gave the isonitrile insertion/addition products [(RCp) 2M( η 2-Me 3C–NC–CC–CH 3)( κ-CN–CMe 3) +] ( 8a– d). Complex 8b was characterized by X-ray diffraction. It contains a η 2-iminoacyl ligand with N-inside orientation and there is a κ- tert-butylisocyanide coordinated to the cationic metallocene framework. Complex 8b exhibits the typical structural characteristics of a d o-configurated isonitrile complex (bond lengths 2.350(4) and 1.148(4) Å for the Zr– C N–R unit). Insertion of 2,6-dimethylphenylisocyanide into the Zr–C(sp) σ-bond of in situ generated [Cp 2Hf–CC–CH 3 +] cation 2d leads to the formation of the analogous cationic hafnocene complex 8e.