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(CH2CMe2Ph)=O}] (Ar = 2,6-Me2C6H3)

Pure imido eta(2)-iminoacyl derivatives [MCpX(NAr) {eta(2)-C(R)=NAr}] (Ar = 2,6-Me2C6H3; M = Nb, Cp = eta(5)-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 = eta(5)-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 eta(2)-alkyliminoacyl [TaCp*Me(NAr){eta(2)-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 eta(2)-methyliminoacyl [TaCp*R(NAr){eta(2)-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 eta(2)-acyl [TaCp*X(NAr){eta(2)-C(R)=O}] (Ar = 2,6-Me2C6H3; X = R = CH2SiMe3, 31; CH2CMe2Ph, 32; CH2CMe3, 33; CH2Ph, 34), chloro eta(2)-acyl (X = Cl, R = CH2CMe2Ph, 35; NMe2, 36) and methyl eta(2)-acyl (X = Me, R = CH2CMe2Ph, 37; CH2CMe3, 38) complexes. Dinuclear ene diolate complexes [{TaCp*X(NAr)}(2){mu-eta(2)-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 eta(2)-iminoacyl complexes [TaCp*X(O){eta(2)-C(R)=NAr}] (X = Cl, R=CH2CMe3, 41; X = R = Ph, 42). All compounds were characterised by NMR (H-1,C-13) spectroscopy and IR measurements. The molecular structures of 1, 7 and 37 were studied by X-ray diffraction methods and are discussed. (C) 2000 Published by Elsevier Science S.A. All rights reserved.