The effect of charge upon CO-adsorption by ionic group 5 and group 9 transition metal clusters

Cold M-n(+/-) group 5 (V, Nb, Ta) and group 9 (Co, Rh, Ir) cationic as well as anionic clusters with up to about 30 atoms were produced by laser vaporization in supersonic expansion, and their reactivity with carbon monoxide was investigated in an FT-ICR mass spectrometer under conditions of bimolecular collisions. Anionic and cationic clusters of the later, group 9 transition metal elements, as well as cations of group 5 exhibit very similar behavior and trends. While the smallest species, n less than or equal to 4 are almost unreactive under the conditions of our experiment, for larger clusters above n = 5 the rate of CO attachment increases abruptly by at least three orders of magnitude. On the other hand, a strikingly different behavior is found for the group 5 anion clusters, V-n(-), Nb-n(-), and Ta-n(-). For these elements, the threshold for anion cluster reactivity is significantly shifted from n = 5 to much larger sizes, with the abrupt rise in reactivity occurring only above n = 14-15. Possible reasons for the drastic difference in reactivity between the earlier and later transition metal anion clusters are discussed. (C) 2003 Elsevier B.V. All rights reserved.