SYNTHESIS, STRUCTURE, AND CHEMISTRY OF HYDRIDE AND ALKYL NIOBOCENE KETENE AND KETENIMINE DERIVATIVES - X-RAY CRYSTAL-STRUCTURE OF B(ETA(5)-C5H4SIME3)(2)(CH3)(ETA(2)-(C,O)O=C=CPH2)]

[Nb(eta(5)-C5H4SiMe3)(2)Cl(eta(2)-(C,Z)-ZCCRPh)] (1a, Z = NPh, R = Ph; 1b, Z = NPh, R = Me; 1c, Z = NPh, R = Et; 1d, Z = NPh, R = H; 2, Z = O, R = Ph) reacts with Li(BEt3H) and with Grignard (RMgI) or dialkyl m agnesium (R2Mg) reagents to give the hydride niobocene complexes [Nb(e ta(5)-C5H4SiMe3)(2) H(eta(2)-(C,Z)-ZCCRPh)] (3a, Z = NPh, R = Ph; 3b, Z = NPh, R = Me; 3c, Z = NPh, R = Et; 3d, Z = NPh, R = H, 4, Z = O, R = Ph) and the alkyl niobocene complexes [Nb(eta(5)-C5H4SiMe3)(2)R(eta( 2)-(C,Z)-ZCCR'Ph) (5a, Z = NPh, R = Me, R' = Ph; 5b, Z = NPh, R = R' = Me; 5c, Z = NPh, R = Me, R' = Et; 5d, Z = NPh, R = Me, R' = H; 6a, Z = O, R = Me, R' = Ph; 6b, Z = O, R = Et, R' = Ph) respectively. The mo lecular structure of 6a was determined by single-crystal diffractometr y. It shows a typical bent-metallocene geometry around the niobium ato m with the eta(2)(C,O) ketene and methyl groups arrayed in the plane b etween the two cyclopentadienyl rings. Finally, some hydride and alkyl niobocene ketenimine and ketene complexes were easily protonated with 1 equiv. of HBF4 . OEt2 giving rise, in one step, to the correspondin g eta(2)-iminoacyl and eta(2)-acyl complexes Nb(eta(5)-C5H4SiMe3)(2)Y( eta(2)-(C,Z)-ZCCRHPh)](+) +[BF4-] (7a, Y = H, Z = NPh, R = Ph; 7b, Y = H, Z = NPh, R = Me; 8a, Y = Me, Z = NPh, R = Ph; 8b, Y = Me, Z = NPh, R = Et; 9, Y = Me, Z = O, R = Ph). H-1 NMR studies on several unsymme trical ketenimine-bearing chloro, hydride and methyl niobocene complex es containing E-Z isomeric mixtures were carried out, and it was found that intermolecular isomerization of the E and Z isomers may govern t he different observed E-Z ratios. The structures of the different fami lies of complexes were determined by spectroscopic methods.