Insertion of carbon dioxide into a rhodium(III)–hydride bond: a theoretical study†

Abstract

The insertion of CO₂ into the Rh^III–H bond of the rhodium dihydride complexes cis-[RhH₂(PH₃)₃]⁺ and cis-[RhH₂(PH₃)₂(H₂O)]⁺ was theoretically investigated by ab initio MO/MP2 and MP4SDQ methods. The transition state (TS) is product-like, in which an η¹-formate anion is almost formed. Its geometry is significantly influenced by the ligand trans to CO₂; the formate is considerably shifted from a position trans to hydride when the latter is trans to CO₂, but only slightly when either PH₃ or H₂O is trans to CO₂. The activation barrier (E_a) and the reaction energy (ΔE) were calculated to be 53.8 and –3.3 kcal mol^–1, respectively, when the hydride ligand is trans to CO₂, 41.7 and –8.0 kcal mol^–1 when PH₃ is trans to CO₂ and 24.0 and –27.0 kcal mol^–1 when H₂O is trans to CO₂, where MP4SDQ values are given and a negative ΔE value indicates that the reaction is exothermic. These results are clearly understood in terms of the trans influence of H (hydride), PH₃ and H₂O.