η1-Vinylidene to η2-Alkyne Isomerization of Tungsten and Molybdenum Complexes

The thermal isomerizations of η1‐vinylidene complexes [(η5‐C5H5)(CO)(NO)W=C=CHR] [5a, R = Si(CH3)2C(CH3)3; 5c, R = C(CH3)3] as well as [(η5‐C5H5)(CO)(NO)Mo=C=CHC(CH3)3] (8), to the corresponding η2‐1‐alkyne complexes [(η5‐C5H5)(CO)(NO)W(η2‐H−C≡C−R)] [7a, R = Si(CH3)2C(CH3)3; 7c, R = C(CH3)3] and [(η5‐C5H5)(CO)(NO)Mo{η2‐H−C≡C−CHC(CH3)3}] (9) has been investigated. Activation parameters for the isomerization of 5a in [D6]benzene and 8 in [D8]toluene and [D5]ethanol were determined. In [D8]toluene η1‐vinylidene complex 5a undergoes a single step 1,2‐shift of the silyl group from Cβ to Cα. However, complex 8 shows dichotomous behavior. The isomerization 8 ⇄ 9, dependent upon the solvent applied, occurs by means of two different pathways: in a nonpolar solvent, 8 tautomerizes via the 1,2‐migration of the hydrogen atom to 9 and in ethanol this tautomerization proceeds by a multi‐step process via deprotonation‐protonation and subsequent reductive elimination. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)