Olefin and CO Competition Equilibria for Bis(cyclopentadienyl)vanadium(II)

The substitution reactions of VCp 2 (CO) with some selected olefins have been studied. Tetracyanoethylene (tcne) and fumaronitrile (fn), containing electron-withdrawing substituents, promptly react with VCp 2 (CO) with quantitative formation of VCp 2 (tcne) and VCp 2 (fn), respectively, while acrylonitrile and diethyl fumarate (defu) gave partial displacement of coordinated carbon monoxide under a CO atmosphere. No CO displacement from VCp 2 (CO) was observed with cyclooctene and norbornene. The carbonylation of VCp 2 (defu) (VCp 2 (defu) + CO ⇄ VCp 2 (CO) + defu) has been studied gas volumetrically from both sides, and the equilibrium constant for the displacement of coordinated defu by carbon monoxide was estimated to be 55 ± 8 at 298.5 K. Variable-temperature experiments between 298.5 and 327.7 K have shown the enthalpy change to be −2.4 kcal mol -1 . The equilibrium constant for the addition of defu to VCp 2 (VCp 2 + defu ⇄ Cp 2 (defu)) has been evaluated spectroscopically to be 150 ± 11 at 298.5 K. From the available data, the equilibrium constant for the addition of carbon monoxide to VCp 2 (VCp 2 + CO ⇄ VCp 2 (CO)) has been calculated to be K a = (8.2 ± 0.4) × 10 3: i . e ., about 2 orders of magnitude higher than that for the complexation of defu. The present data for the 3d 3 system of vanadium(II) (ν̃ CO in toluene 1881 cm -1 ) are compared with those exhibited by the 5d 10 system of AuCl(CO) (ν̃ CO in toluene 2153 cm -1 ), whose behavior consistently shows an opposite dependence, as far as the olefin/CO competition is concerned, on the nature of the olefin.