Reaction of [Rh(acac)(cod)] with 1 equivalent of HClO 4 and subsequent treatment with tetramethylammonium or cesium salts of [7-SR-8-R′-7,8-C 2 B 9 H 10 ]anions affords [Rh(7-SR-8-R′-7,8-C 2 B 9 H 10 )-(cod)] (1: R, R′ ) Ph; 2: R ) Ph, R′ ) Me; 3: R ) Et, R′ ) Me). The structure of 1 has been determined by crystallographic studies. The Rh(I) has a normal four-coordinated square planar geometry and the carborane ligand is bonded to the metal by means of a S-Rh and a B(11)-H-Rh bond. The cod ligand fulfills the coordination sphere of the metal with two olefin-Rh bonds. Variable-temperature 1 H{ 11 B} NMR experiments indicate that 1 undergoes B-H/B-H-Rh exchange coupled with rotation of the [Rh(cod)] + moiety. This dynamic process appears to involve exclusively the B( 11)-H group. The presence of two species that exchange at room temperature is detected for complexes 2 and 3. A structure analogous to 1 is suggested for the major species, whereas the minor species appears to involve a [Rh(cod)] + fragment bonded to the cage by a single S-Rh bond. Complexes 1-3 rearrange in solution to give [closo-3-(η 3 -C 8 H 13 )-1-SR-2-R′-3,1,2-RhC 2 B 9 H 9 ] (4: R, R′ ) Ph; 5: R ) Ph, R′ ) Me; 6: R ) Et, R′ ) Me). The crystal structure of 4 reveals lack of bonding between C(1)-C(2) and the cluster adopts a pseudocloso geometry. The weighted average 11 B{ 1 H} NMR chemical shift of 5 and 6 suggests that these clusters have a normal closo structure. Kinetic studies of the reactions 2 f 5 and 3 f 6 showed a common first-order rate constant for the two reactions. Complexes 2 and 3 in the presence of 1 equivalent of PPh 3 present similar activity in the hydrogenation of cyclohexene to the parent diphosphine complexes [Rh(7-SR-8-R′-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ]. The closo-cyclooctenyl complexes 4 and 5 exhibit higher activity than the parent exo-nido isomers in the hydrogenation of cyclohexene.

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