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.