EFFECT OF SOLVENT AND PRESSURE ON THE REACTIVITY OF PHOTOPRODUCED CR(CO)5, MO(CO)5, W(CO)5 TRANSIENTS, AS REVEALED BY THE OBSERVED QUANTUM YIELDS FOR THE PHOTOSUBSTITUTION OF CR(CO)6, MO(CO)6, W(CO)6

The effect of solvent and pressure up to 200 MPa on the quantum yield for photosubstitution of CO was studied for the hexacarbonyls of chromium, molybdenum, and tungsten. The employed solvents include different n-alkanes (ranging from n-pentane to n-dodecane), the very weakly coordinating solvent perfluorohexane, and acetonitrile. The M(CO)5(solvent) intermediates were trapped with piperidine, pyridine, and acetonitrile. The quantum yields for CO substitution in Cr(CO)6 at ambient conditions vary between 0.47 ± 0.02 and 0.73 ± 0.01, depending on the employed solvent and ligand. For the photosubstitution of M(CO)6 by pyridine in n-heptane, the quantum yield decreases along the series Mo > W > Cr (0.93, 0.79, and 0.72, respectively) at ambient conditions. For all the investigated systems the quantum yields decrease with increasing pressure, and the corresponding volumes of activation (ΔV*Φ/(1-Φ)) vary between +5 and +14 cm3 mol-1. Different models are discussed to account for the experimental data. © 1990 American Chemical Society.