The compound CpRh(C2H3CO2tBu)21 has been synthesised as a mixture of two pairs of interconverting isomers which differ in the relative orientations of the alkene substituents. The four isomers have been fully characterised by NMR spectroscopy. When complex 1 is photolysed in the presence of a silane, HSiR2R′ [R2R′ = Et3, Me3, HEt2, (OMe)3 and Me2Cl] the corresponding Si–H oxidative addition products CpRh(SiR2R′)(H)(C2H3CO2tBu) and CpRh(H)2(SiR2R′)2 are formed. The Rh(III) complexes CpRh(SiR2R′)(H)(C2H3CO2tBu) exist in two isomeric forms of comparable energy which interconvert in an intramolecular process that does not involve a reversible [1,3] hydride or [1,3] silyl migration. The hydride 1H NMR resonances for these species consequently broaden before coalescing into a single peak. For R2R′ = Et3, the activation parameters for interchange from the major to minor isomer were ΔH‡ = 60.2 ± 2 kJ mol−1 and ΔS‡ = 8 ± 9 J mol−1 K−1, while for R2R′ = Me3 and Et2H, ΔH‡ = 61.5 ± 1 kJ mol−1, ΔS‡ = 6 ± 5 J mol−1 K−1, and ΔH‡ = 61.8 ± 3 kJ mol−1, ΔS‡ = 12 ± 9 J mol−1 K−1 respectively for conversion from the major isomer to the minor. For these complexes an η2-Rh–H–Si transition state or intermediate is consistent with the evidence. When R2R′ = (OMe)3 and Me2Cl the change in appearance of the hydride resonances is more complex, with the activation parameters for interchange from the major to minor isomer for the former species being ΔH‡ = 78.3 ± 2 kJ mol−1 and ΔS‡ = 30 ± 7 J mol−1 K−1 while for Me2Cl the barrier proved too high to measure before decomposition occurred. The complex spectral changes could be simulated when a discrete η2-Rh–H–Si intermediate was involved in the isomer interconversion process and hence silane rotation in all these systems is proposed to involve two isomers of CpRh(η2-HSiR2R′)(C2H3CO2tBu).
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