Multiphoton ionization of group VIB hexacarbonyl Van Der Waals clusters: trends in intracluster photochemistry

Van der Waals clusters of M(CO)₂ (M = Cr, Mo, W) generated in the free-jet expansion of a pulsed beam of seeded helium are subjected to multiphoton ionization (MPI) and the product ions analyzed by quadrupole mass spectrometry. We previously reported the observation of efficient production of MoO⁺ and MoO₂⁺ following of MPI of Mo(CO)₆ van der Waals clusters, and proposed that these ions arise through novel reactions between a neutral photoproduced metal atom and the ligands of an adjacent metal carbonyl "solvent" molecule within the cluster. In order to test some of the predictions of this model, we have now examined the MPI of van der Waals clusters of the other Group VIB hexacarbonyls. We find the same novel behavior (viz. efficient production of metal oxide ions) in the W(CO)₆ system as that previously observed for Mo)CO)₆. However, we find no evidence of such behavior in the Cr(CO)₆ system. Based on these observations, we suggest that the reactivity of first-row transition metal atoms may be fundamentally different from that of second- or third-row metals. These differences are discussed in terms of the occupancy and relative size of the metal d orbitals.