η 1 ‐Vinylidene to η 2 ‐Alkyne Isomerization of Tungsten and Molybdenum Complexes

The thermal isomerizations of η 1 ‐vinylidene complexes [(η 5 ‐C 5 H 5 )(CO)(NO)W=C=CHR] [ 5a , R = Si(CH 3 ) 2 C(CH 3 ) 3 ; 5c , R = C(CH 3 ) 3 ] as well as [(η 5 ‐C 5 H 5 )(CO)(NO)Mo=C=CHC(CH 3 ) 3 ] ( 8 ), to the corresponding η 2 ‐1‐alkyne complexes [(η 5 ‐C 5 H 5 )(CO)(NO)W(η 2 ‐H−C≡C−R)] [ 7a , R = Si(CH 3 ) 2 C(CH 3 ) 3 ; 7c , R = C(CH 3 ) 3 ] and [(η 5 ‐C 5 H 5 )(CO)(NO)Mo{η 2 ‐H−C≡C−CHC(CH 3 ) 3 }] ( 9 ) has been investigated. Activation parameters for the isomerization of 5a in [D 6 ]benzene and 8 in [D 8 ]toluene and [D 5 ]ethanol were determined. In [D 8 ]toluene η 1 ‐vinylidene complex 5a undergoes a single step 1,2‐shift of the silyl group from C β to C α . 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)