Laser-ablated palladium atoms react with acetylene in excess argon to form the strong Pd-η 2 -(C 2 H 2 ) and Pd-η 2 -(C 2 H 2 ) 2 π 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 ( 13 C 2 H 2, C 2 D 2, C 2 HD) and density functional theory (DFT) isotopic frequency calculations. The Pd(C 2 H 2 ) 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-η 2 -(C 2 H 2 ) 2 is observed at 1765 cm -1 . The Pd 2 -η 2 -(C 2 H 2 ) complex produced from the reaction of two Pd atoms with C 2 H 2 is characterized by a C−C mode at 1566 cm -1 . The interaction between atomic Pd and C 2 H 2 involves a small amount of charge transfer based on NBO analysis, which is enhanced by a Pd 2 dimer coordinated to C 2 H 2 . 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−C 2 H 2 interaction culminating in acetylene chemisorbed on palladium.
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Wang et al. (2002) studied this question.