Laser-ablated palladium atoms react with acetylene in excess argon to form the strong Pd-eta(2)-(C2H2) and Pd-eta(2)-(C2H2)(2) pi complexes. The C-C and C-H stretching modes and two C-H deformation modes are observed in the matrix infrared spectrum and identified through isotopic substitution ((C2H2)-C-13, C2D2, C2D2) and density functional theory (DFT) isotopic frequency calculations. The Pd(C2H2) complex is characterized by a C-C stretching mode near 1710 cm(-1) and a 39.8 kcal/mol binding energy predicted by DFT. The antisymmetric C-C stretching mode for Pd-eta(2)-(C2H2)(2) is observed at 1765 cm(-1). The Pd-2-eta(2) -(C2H2) complex produced from the reaction of two Pd atoms with C2H2 is characterized by a C-C mode at 1566 cm(-1). The interaction between atomic Pd and C2H2 involves a small amount of charge transfer based on NBO analysis, which is enhanced by a Pd-2 dimer coordinated to C2H2. As a result, Pd-2 dimer can reduce the C-C triple bond to a double bond. These complexes represent first steps in the Pd-C2H2 interaction culminating in acetylene chemisorbed on palladium.