Inverse Isotope Effects in Single-Crystal to Single-Crystal Reactivity and the Isolation of a Rhodium Cyclooctane σ-Alkane Complex

[Image: see text] The sequential solid/gas single-crystal to single-crystal reaction of [Rh(Cy(2)P(CH(2))(3)PCy(2))(COD)][BAr(F)(4)] (COD = cyclooctadiene) with H(2) or D(2) was followed in situ by solid-state (31)P{(1)H} NMR spectroscopy (SSNMR) and ex situ by solution quenching and GC-MS. This was quantified using a two-step Johnson–Mehl–Avrami–Kologoromov (JMAK) model that revealed an inverse isotope effect for the second addition of H(2), that forms a σ-alkane complex [Rh(Cy(2)P(CH(2))(3)PCy(2))(COA)][BAr(F)(4)]. Using D(2), a temporal window is determined in which a structural solution for this σ-alkane complex is possible, which reveals an η(2),η(2)-binding mode to the Rh(I) center, as supported by periodic density functional theory (DFT) calculations. Extensive H/D exchange occurs during the addition of D(2), as promoted by the solid-state microenvironment.