Metal cation-acetylene complexes (M = V, Fe, Co, Ni) are produced in mol. beams and studied with IR photodissocn. spectroscopy in the C-H stretching region. Each complex has two vibrational bands corresponding to the sym. and asym. stretches of acetylene that are shifted to the red of these vibrations in the isolated acetylene mol. D. functional theory reveals the sources of the red shifted vibrations and their relative magnitudes. Fe+, Co+, and Ni+ form pi-complexes with acetylene, while V+(C2H2) is a metallacycle. [on SciFinder (R)]