On the structure, carbonyl-stretching frequencies and relative stability of trans- and cis-[W(CO)4(η2-alkene)2]0/+: a theoretical and IR spectroelectrochemical study

Abstract

The geometries, ν(CO) frequencies, relative enthalpies and Gibbs energies of the alkene carbonyl complexes [W(CO)₄(η²-C₂H₄)₂]^0/+ and [W(CO)₅(η²-C₂H₄)] were calculated by means of the GAUSSIAN 98 program using the hybrid B3-LYP density functional. The predicted geometries and ν(CO) vibrational frequencies agree with the experimental data. The calculated relative energies (ΔG₂₉₈) show that trans-[W(CO)₄(η²-C₂H₄)₂] is more stable by 10 or 12 kJ mol⁻¹ (depending on the basis set applied) than cis-[W(CO)₄(η²-C₂H₄)₂]. In contrast to this, the stability of their one-electron oxidation products, the corresponding 17-electron cationic complexes, is reversed, the cationic form of the cis isomer being preferred by 14 or 10 kJ mol⁻¹. Comparison of the calculated and experimental vibrational spectra has elucidated the electrochemical oxidation path of trans-[W(CO)₄(η²-alkene)₂] compounds. The electrochemical oxidation of trans-[W(CO)₄(η²-1-butene)₂] produces the corresponding 17-electron cation, which undergoes spontaneous isomerisation to cis-[W(CO)₄(η²-1-butene)₂]⁺. The identity of the latter species has been established by cyclic voltammetric and IR spectroelectrochemical experiments at low temperature.