Cis– trans isomerism in [M(CO) 2I 4] − (M=Rh, Ir): Kinetic, mechanistic and spectroscopic studies

Abstract

Infrared spectroscopic studies show that the cis– trans isomerisation of [Rh(CO) 2I 4] − ( 1) reaches an equilibrium in solution favouring the trans isomer ( K eq≈10 in CH 2Cl 2 at 25°C). The approach to equilibrium obeys a first order rate law with a half life of ca. 40 min at 25°C. Activation parameters for the isomerisation are Δ H ‡ 103 (±3) kJ mol −1, Δ S ‡ +32 (±9) J mol −1 K −1 in CH 2Cl 2 and Δ H ‡ 99 (±2) kJ mol −1, Δ S ‡+25 (±4) J mol −1 K −1 in THF. In polar coordinating solvents, formation of monocarbonyl complexes [Rh(CO)I 4(sol)] − occurs in competition with cis– trans isomerisation, and in the presence of added iodide salt, [Rh(CO)I 5] 2− is formed. Isotopic labelling experiments show that added 12CO is incorporated into trans- 1 on isomerisation of cis-[Rh( 13CO) 2I 4] −. Addition of CO to the monocarbonyls, [Rh(CO)I 4(sol)] − and [{Rh(CO)I 4} 2] 2− gives predominantly trans-[Rh(CO) 2I 4] −. A mechanism is proposed for cis– trans isomerisation involving dissociation of a CO ligand. Elemental and infrared spectroscopic analysis of crystals obtained from the residues of carbonylation experiments on [Ir(CO) 2I 3(Me)] − are consistent with a new trans isomer of [Ir(CO) 2I 4] −. The same species is formed in the reaction of [{Ir(CO)I 4} 2] 2− with CO. Isomerisation to cis-[Ir(CO) 2I 4] − only occurs in the presence of added CO. The ν(CO) frequencies for isotopomers of cis- and trans-[M(CO) 2I 4] − (M=Rh, Ir) are analysed using C–O factored force-fields. The force constants obtained indicate stronger metal–CO back-donation for the cis relative to the trans isomers and also for iridium relative to rhodium.