MULTIPLE METAL-CARBON BONDS .13. PREPARATION AND CHARACTERIZATION OF MONOCYCLOPENTADIENYL MONONEOPENTYLIDENE COMPLEXES OF NIOBIUM AND TANTALUM INCLUDING THE 1ST DETAILS OF AN ALPHA-ABSTRACTION PROCESS

The neopentyl complexes, Ta(CH2CMe3)2X3 (X = Cl or Br), react with TlC5H5 or LiC5Me5 to give the monocyclopentadienyl complexes, Ta(η5-C5R5)(CH2CMe 3)2X2 (R = H or Me). Several of these decompose smoothly to give neopentane and the electron-deficient neopentylidene complexes, Ta(η5-C5R5)(CHCMe3)X 2. In Ta(η5-C5R5)(CHCMe3)X2 attraction of the CHα electrons by Ta cooperates with the steric effect of the tert-butyl substituent to give large M=Cα-Cβ angles (∼165°) and low values for 1JCHα (∼75-85 Hz) and vCHα (∼2500 cm-1). We propose that the electrophilic d0 metal in Ta(η5-C5R5)(CH2CMe 3)2X2 similarly, but more weakly, interacts with a CHα electron pair in one neopentyl ligand and thereby sets up Hα for abstraction by the second neopentyl ligand. We show that (1) the α-abstraction reaction is intramolecular; (2) the rate of decomposition of TaCp(CH2CMe3)2Cl2 at 309 K varies with solvent in the order ether (1) < pentane (2) < benzene (4) < chloroform (15) ∼ dichloromethane; (3) the rate of decomposition of TaCp(CH2CMe3)2Br2 is 200-400 times that of TaCp(CH2CMe3)2Cl2; (4) the rate of decomposition of a η5-C5H5 complex is 5 × 103 times that of the corresponding η5-C5Me5 complex; (5) the deuterium isotope effect in the rate-determining α-hydrogen abstraction step is approximately 6. Many of the results can be explained adequately if only cis-Ta(η5-C5R5)(CH2CMe 3)2X2 is prone to α-abstraction and trans-Ta(η5-C5R5)(CH2CMe 3)2X2 is inert. This postulate is fully supported by studies of the TaCp(CH2CMe3)(CH2Ph)X2 complexes where the cis/trans equilibrium is observable. The yields of analogous Nb neopentylidene complexes are low. TaCp(CHCMe3)Cl2 reacts quantitatively at 25°C with acetonitrile or benzonitrile to give imido complexes, TaCp[N(R)C=CHCMe3]Cl2, and with diphenylacetylene quantitatively at 25°C to give a new alkylidene complex, TaCp[C(Ph)(PhC=CHCMe3)]Cl2. © 1979 American Chemical Society.