Chemical behaviour of alkyl imido cyclopentadienyl niobium and tantalum(V) complexes in insertion processes. X-ray crystal structures of [MCpCl(NAr){η2-C(Me)NAr}] (Ar=2,6-Me2C6H3; M=Nb, Cpη5-C5H4SiMe3; M=Ta, Cp=η5-C5Me5) and [Ta(η5-C5Me5)Me(NAr){η2-C(CH

Pure imido η2-iminoacyl derivatives [MCpX(NAr){η2-C(R)NAr}] (Ar=2,6-Me2C6H3; M=Nb, Cp=η5-C5H4SiMe3=Cp′, X=Cl, R=Me, 1; X=Me, R=NMe2, 2; Me, 3; X=R=CH2SiMe3, 4; CH2CMe3, 5; CH2Ph, 6; M=Ta, Cp=η5-C5Me5=Cp*, X=Cl, R=Me, 7; CH2SiMe3, 8; CH2CMe2Ph, 9; CH2CMe3, 10; CH2C6H5, 11; 2-(CH2NMe2)C6H4, 12; NMe2, 13; X=R=CH2SiMe3, 14; CH2CMe2Ph, 15; CH2CMe3, 16; CH2C6H5, 17; C6H5, 18) can be prepared by reaction of chloro alkyl or dialkyl imido complexes MCpXR(NAr) with 1 equivalent of 2,6-Me2C6H3NC. Reaction of alkyl methyl imido complexes TaCp*RMe(NAr) with 1 equivalent of isocyanide 2,6-Me2C6H3NC takes place with migration of alkyl or methyl group, leading to the formation of a mixture of methyl η2-alkyliminoacyl [TaCp*Me(NAr){η2-C(R)NAr}] (Ar=2,6-Me2C6H3; R=CH2SiMe3, 19; CH2Ph, 21; CH2CMe2Ph, 23; CH2CMe3, 25; C6H5, 27; 2-(CH2NMe2)C6H4, 29; NMe2, 30) and alkyl η2-methyliminoacyl [TaCp*R(NAr){η2-C(Me)NAr}] (Ar=2,6-Me2C6H3; R=CH2SiMe3, 20; CH2Ph, 22; CH2CMe2Ph, 24; CH2CMe3, 26; C6H5, 28) complexes. Alkyl imido derivatives also react with CO to give alkyl η2-acyl [TaCp*X(NAr){η2-C(R)O}] (Ar=2,6-Me2C6H3; X=R=CH2SiMe3, 31; CH2CMe2Ph, 32; CH2CMe3, 33; CH2Ph, 34), chloro η2-acyl (X=Cl, R=CH2CMe2Ph, 35; NMe2, 36) and methyl η2-acyl (X=Me, R=CH2CMe2Ph, 37; CH2CMe3, 38) complexes. Dinuclear ene diolate complexes [{TaCp*X(NAr)}2{μ-η2-OC(R)C(R)O}] (Ar=2,6-Me2C6H3; X=Cl, R=CH2CMe3, 39; X=Me, R=2-(CH2NMe2)C6H4, 40) are formed by reaction of the chloro and methyl alkyl imido derivatives with carbon monoxide, whereas using [TaCp*XR(NAr)] (Ar=2,6-Me2C6H3; X=Cl, R=CH2CMe3; X=R=Ph) a different sequence of reactions takes place leading finally to oxo η2-iminoacyl complexes [TaCp*X(O){η2-C(R)NAr}] (X=Cl, R=CH2CMe3, 41; X=R=Ph, 42). All compounds were characterised by NMR (1H, 13C) spectroscopy and IR measurements. The molecular structures of 1, 7 and 37 were studied by X-ray diffraction methods and are discussed.