Metal cation-acetylene complexes (M = V, Fe, Co, Ni) are produced in molecular beams and studied with infrared photodissociation spectroscopy in the C-H stretching region. Each complex has two vibrational bands corresponding to the symmetric and asymmetric stretches of acetylene that are shifted to the red of these vibrations in the isolated acetylene molecule. Density 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.