Olefin and CO competition equilibria for Bis(cyclopentadienyl)vanadium(II)

The substitution reactions Of VCp2(CO) with some selected olefins have been studied. Tetracyanoethylene (tcne) and fumaronitrile (fn), containing electron-withdrawing substituents, promptly react with VCp2(CO) with quantitative formation Of VCp2(tcne) and VCp2(fn), respectively, while acrylonitrile and diethyl fumarate (defu) gave partial displacement of coordinated carbon monoxide under a CO atmosphere. No CO displacement from VCp2(CO) was observed with cyclooctene and norbornene. The carbonylation Of VCp2(defu) (VCp2(defu) + CO reversible arrow VCp2(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 VCp2 (VCp2 + defu reversible arrow 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 VCp2 (VCp2 + CO reversible arrow VCp2(CO)) has been calculated to be K-a = (8.2 +/- 0.4) x 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) ((nu) over tilde (CO) in toluene 1881 cm(-1)) are compared with those exhibited by the 5d(10) system of AuCl(CO) ((nu) over tilde (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.