Ultraviolet photoelectron spectra for WnN2- (n=1-8) clusters produced by addition of atomic and molecular nitrogen on W anion clusters are presented. Evidence is provided that molecular chemisorption of N-2 is more stable than the dissociative one on tungsten anion clusters consisting of eight atoms or less, which is completely different from the results on tungsten bulk surfaces. A general tendency toward molecular chemisorption for small clusters can be explained by reduced charge transfer from the metallic d-orbitals of cluster in contrast to bulk d-orbitals. Comparative studies on chemisorption of atomic and molecular species of diatomic molecules on nanoclusters are expected to provide important information on size-dependent changes of chemical properties of nanoclusters. (C) 2003 American Institute of Physics.