the ESR spectrum was recorded at increasing temperatures (-40 to +25 °C). After the experiment, the mother solution was evaporated to dryness at 25 °C. No phosphine oxide was detected.

Reaction of (7i-C5H5)Re(NO)(PPh3)(CH2C6H5) (1) with Ph3C+PF6~a t -78 °C gives the benzylidene sc-[(?;-C5H5)Re(NO)(PPh3)(=CHC6H5)]+PF6" (2k), which isomerizes upon warming (tl/2 = 17 min at 29.5 °C, AH* = 20.9 ± 0.4 kcal/mol, AS" = -3.8 ± 0.2 eu) to a new Re=C geometric isomer, ac-[(j7-C5H5)Re(NO)(PPh3)(=CHC6H5)]+PF6' (2t). The 2t/2k equilibrium mixture is >99:1, but irradiation between -78 and -20 °C establishes a (55 ± 3):(45 ± 3) photostationary state. The structures of 2t and 2k are confirmed by X-ray crystallography (2t) and extended Hückel MO calculations. Nucleophiles (Nu) Li(C2H5)3BD, CH3Li, CH3CH2MgBr, C6H5CH2MgCl, PMe3, and CH3ONa attack 2t to give diastereomerically pure adducts (i)-C5H5)Re(NO)(PPh3)(CHNuC6H5), in which a new chiral center is stereospecifically generated. The same nucleophiles react stereoselectively with 2k, affording adducts in (generally) 92-95:8-5 diastereomer ratios. The minor diastereomers from 2k are identical with the products from 2t. Thus nucleophilic attack upon 2t and 2k occurs preferentially from the same direction.An X-ray structure of (SS,RR)-(?j-C5Hj)Re(NO)(PPh3)(CH(CH2C6H5)C6H5) (5t, from C6H5CH2MgCl attack upon 2t) establishes this direction to be antiperiplanar to the PPh3. When (5,5,RR)-( 7)-C5H5)Re(NO)(PPh3)(CHDC6H;) ( ---t;from Li(C2H5)3BD attack upon 2t) is treated with Ph3C+PF6", exclusive H~abstraction occurs to give first 2k-a-dx and then 2t-a-di. When (57? 5>( -£5 5) ß( )( 3)(CHDC6H5) (l-a-rf,-k; from Li(C2H5)3BH attack upon 2t-a-dx or Li(C2H5)3BD attack upon 2k) is treated with Ph3C+PF6", preferential (92:8) D" abstraction occurs to give undeuterated 2k and (subsequently) 2t. These data (after an isotope effect correction) indicate that the pro-R -hydrogen of 1 is abstracted by Ph3C+PF6™ essentially stereospecifically. Insights into this specificity are gained by *H NMR, extended Hückel MO calculations, and the chemospecific and stereospecific abstraction of CH3Ó~f rom (j^C5H5)Re(NO)(PPh3)(CH(OCH3)C6H5) (SRRS: 7t; SSRR: 7k) by Ph3C+PF6™.A mechanism is proposed in which the least stable rotamer of 1 is the most reactive toward Ph3C+PF6~.(1) (a) UCLA, (b) University of Michigan.

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Reaction of [ (a5-C5H5)Re(NO)(PPh3)(=CH,)]+PF6-(1, -78 "C) with CH3SCH3 gives sulfonium salt [ ( q5-C5H5)Re( NO) ( PPh3) (CH2S( CH3),)]+PF6-(2, 90-95 % ). Variable-temperature NMR data show that CH,SCH, reversibly gives from 2 with AG*262 = 14.2 kcal/mol and AC*zss = 12.9 kcal/mol. Reaction of 2 with PPh, and pyridine give [ (q5-C5H5)Re(NO)(PPh3)(CH2PPh3)]+PF6-(4, 76%) and [(q5-C5H5)Re-(NO)(PPh3)(CH,N(CH),CH)]+PF6-( 5 ) , respectively. Reaction of 2 with RS-gives (q5-C5H5)Re(NO)-(PPh,)(CH,SR) (6,79-95%; R = CH, (a), C6H5 (b), CH2C6H5 (c)). Reaction of 1 or 2 with 6a gives binuclear sulfonium salt [ (q5-C5H5)Re(NO)(PPh3)(CH2-)j2S+CH3 PF6-(7+PF6-, 89-92%) as a 2:l:l mixture of diastereomers. Treatment of 7+PF6-with NaI/CH3CN gives 7+I-, which crystallizes as one diastereomer as shown by X-ra crystallography. Crystal data: orthorhombic, Pnma; a = 18.348 (6) A, b = 20.767 (2) A, c = 13.944 (2) $I; 2 = 4. Reaction of 7+PF6-with PPh, gives 4 (90%) and 6a (71%). Pyrolysis of 7+PF6-(80 OC, 8 h) gives ($-C,H,)Re(NO)(PPh,)(CH,) (8, 84%) and [(q5-C5H5)Re(NO)(PPh3)(=CHSCH3)]+PF; (9,82% 1. Reaction of 1 with Na2S gives trinuclear sulfonium salt [ (I~~-C~H~)R~(NO)(PP~,)(CH,-)],SPF~ (10, 84%). Complex 10 reacts only sluggishly with PPh, (65 OC). These reactions show that the (7 -C,H5)Re(NO)(PPh3) moiety significantly enhances the basicity of sulfide sulfur atoms that are p to it.

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