Photochemical studies of M(CO)6 (M = Cr,Mo, W) at low temperature in solution. Infrared spectra of M(CO)5 (solvent) (solvent = methylcyclohexane,methylene chloride) and W(CO)5L (L = aromatic hydrocarbon)

The infrared spectra of M(CO)₅(MCH) (MCH = methylcyclohexane; M = Cr, Mo, W), formed by 366 nm irradiation of M(CO)₆ at ?78°C in rigorously purified methylcyclohexane, are reported. The previously reported spectrum of “W(CO)₅” at low temperature in methylcyclohexane/isopentane solution is attributed to W(CO)₅(impurity), where the impurity is probably an aromatic or olefinic hydrocarbon. Spectra in methylene chloride solution are also discussed. The photochemical reactions of W(CO)₆ with aromatic hydrocarbon ligands in methylcyclohexane solution were also studied at ?78°C in a low temperature infrared cell. Irradiation (366 nm) of W(CO)₆ at ?78°C in rigorously purified methylcyclohexane solution containing approximately 5% (v/v) toluene, benzene, mesitylene, biphenyl, or p-xylene initially produces the complex W(CO)_5? (MCH). In the presence of the aromatic hydrocarbon, this complex is unstable and it decomposes in a dark reaction to give a complex which has an infrared spectrum typical for a C_4v M(CO)₅X molecule. It is proposed that the product of the dark reaction is W(CO)₅(aromatic), formed by reaction of W(CO)₅(MCH) with the aromatic ligand in solution. The infrared spectra of the W(CO)_5? (aromatic) complexes are different from the spectra previously reported for these complexes. It is shown that the spectra previously reported for W(CO)_5? (aromatic) are actually attributable to W(CO)₅(hexane) (hexane was the solvent used in the previous study); these spectra were probably obtained before W(CO)₅(hexane) had time to react with the aromatic hydrocarbon.