PRIMARY PHOTOPRODUCTS IN THE PICOSECOND TIME DOMAIN FROM PHOTOLYSIS OF HEXACARBONYL AND AMINOPENTACARBONYL TUNGSTEN(0) COMPLEXES

The photolysis of W(CO)5L (L = pyridine, piperidine) produces solvo complex products represented as W(CO)5S in times of less than 20 ps. When the solvents present alkyl groups to the W(CO)5 fragment, the transient spectra are similar to that of W(CO)5S in cyclohexane. However, when conventional donor groups, such as alkene and alcohol hydroxyl, are present there is also evidence of the direct coordination of these donors. In some cases, the rearrangement from the alkyl to the more stable complex can be analysed. The time constant for rearrangement of the alkyl-coordinated 1-hexene to the alkene-coordinated form is approximately 600 ps. The distribution of primary products varies as a function of the solvent (1-hexene and the series of alcohols methanol, butanol, octanol). The distribution also changes as a function of the leaving group (CO, piperidine, pyridine). This selectivity is discussed in terms of the Hollebone selection rules for a fast relaxation process and the model proposed by Wang, Zhu and Spears for the solvation of the Cr(CO)5 fragment. As activated W(CO)5 relaxes and acquires a sixth ligand, it appears that two pathways are open. One pathway exploits a fragment of approximately C4v symmetry. The high symmetry is lost on the other pathway. The selectivity is counter to expectation for classical dissociative substitutions and the sense in which these processes are not dissociative is considered.