Synthesis and properties of ((eta-C/sub 5/H/sub 5/)Re(NO)(PPh/sub 3/)(==CHC/sub 6/H/sub 5/)/sup +/PF/sub 6//sup -/: a benzylidene complex that is formed by a stereospecific. cap alpha. -hydride abstraction,e xists as two geometric isomers, and undergoes stereospecific nucleophilic attack

Reaction of (eta-C/sub 5/H/sub 5/)Re(NO)(PPh/sub 3/)(CH/sub 2/C/sub 6/H/sub 5/) (1) with Ph/sub 3/C/sup +/PF/sub 6//sup -/ at -78/sup 0/C gives the benzylidene sc-((eta-C/sub 5/H/sub 5/)Re(NO)(PPh/sub 3/)(==CHC/sub 6/H/sub 5/)/sup +/PF/sub 6//sup -/ (2k), which isomerizes upon warming (t/sub 1/2/ = 17 min at 29.5/sup 0/C, ..delta..H/sup + +/ = 20.9 +- 0.4 kcal/mol, ..delta..S/sup + +/ = 3.8 +- 0.2 eu) to a new Re==C geometric isomer, ac-((eta-C/sub 5/H/sub 5/)Re(NO)(PPh/sub 3/)(==CHC/sub 6/H/sub 5/))/sup +/PF/sub 6//sup -/ (2t). The 2t/2k equilibrium mixture is greater than or equal to 99:1, but irradiation between -78 and -20/sup 0/C establishes a (55 +- 3):(45 +- 3) photostationary state. The structures of 2t and 2k are confirmed by x-ray crystallography (t) and extended Hueckel MO calculations. Nucleophiles (Nu) Li(C/sub 2/H/sub 5/)/sub 3/BD, CH/sub 3/Li, CH/sub 3/CH/sub 2/MgBr, C/sub 6/H/sub 5/MgCl, PMe/sub 3/, and CH/sub 3/ONa attack 2t and 2k to give diastereomerically pure adducts (eta-C/sub 5/H/sub 5/)Re(NO)(PPh/sub 3/)(CHNuC/sub 6/H/sub 5/), in which a new chiral center is sterospecifically generated. 7 figures, 6 tables.