The first terminal organometallic alkylphosphanylidenetantalum(V) complexes [Cp*Ta{1,2-(NSiMe 3 ) 2 C 6 H 4 }(PR)] were obtained with cyclohexyl (2) and isopropyl groups (3) at phosphorus, whereas adamantyl and tert-butyl substituents resulted in the formation of the paramagnetic tantalum(IV)[a] 3137 complex [Cp*Ta{1,2-(NSiMe 3 ) 2 C 6 H 4 }Cl] (4). DFT studies showed that the terminal cyclohexyl and isopropyl phosphanylidene complexes are stable towards dimerization and dissociation.= Ph, cyclohexyl (Cy), tBu] [3e] and [Ta(PPh){OSi-(tBu) 3 } 3 ], [6a] were reported in 1993 and 1994, respectively. Subsequent reactions include phospha-Wittig reactions [3e] and insertion of acetone. [6c]