Insertion of carbon dioxide into a rhodium(III)–hydride bond: a theoretical study†
Abstract
The insertion of CO2 into the RhIII–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 (Ea) 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.