MECHANISM OF ISOMERIZATION OF ALKYLIDENE COMPLEXES [(ETA-5-C5H5)RE(NO)(PPH3)(=CRCHR'R'')]+X- TO ALKENE COMPLEXES [(ETA-5-C5H5)RE(NO)(PPH3)(RHC=CR'R'')]+X- - AN ORGANOMETALLIC WAGNER-MEERWEIN-TYPE REARRANGEMENT

Propylidene complex [(eta-5-C5H5)Re(NO)(PPh3)(= CHCH2CH3)]+PF6- (1c+PF6-) cleanly rearranges to propene complex [(eta-5-C5H5)Re(NO)(PPh3)(H2C = CHCH3)]+PF6- (C6D5Cl, 65-86-degrees-C; DELTA-H = 27 +/-1 kcal/mol; DELTA-S = 3 +/- 3 eu). Experiments with deuterated 1c+PF6- show a modest primary kinetic deuterium isotope effect (k(= CHCH2CH3)/k(= CHCD2CH3) = 1.95-1.40) but a considerable inverse secondary kinetic deuterium isotope effect (k(Re = CH)/k(Re = CD) = 0.50-0.58). A crossover experiment shows intramolecular hydride migration and the absence of PPh3 ligand dissociation. Optically active 1c+PF6 rearranges with retention of configuration at rhenium, but the hydride shift is not stereospecific at the migration terminus. However, the rearrangement of isobutylidene complex [(eta-5-C5H5)Re(NO)(PPh3)(= CHCH(CH3)2)]+SO3F- to isobutylene complex [(eta-5-C5H5)Re(NO)(PPh3)(H2C = C(CH3)2)]+SO3F- is highly stereoselective at the migration origin and terminus, and much faster than that of 1c+PF6- (CD2Cl2, 3-25-degrees-C; DELTA-H = 21 +/- 1 kcal/mol; DELTA-S = 3 +/- 3 eu). Rearrangements of analogous pentylidene and cyclopentylidene complexes are also studied. An orbital symmetry analysis of these reactions is given.