Infrared spectra and structures of nickel and palladium dinitrosyl complexes isolated in solid argon

The infrared spectrum of nickel and palladium dinitrosyls formed by reaction of ground-state Ni or Pd atoms with NO during condensation in solid argon has been reinvestigated, and the energetic, structural, and spectroscopic properties of these compounds have been calculated using density functional theory. The Pd(eta (1)-NO)(2) molecule, already characterized in reactions of laser-ablated Pd, is the only product formed in the reaction between one Pd atom and two nitric oxide molecules. With Ni, in contrast, two different isomeric forms are evidenced, Ni (eta (1)-NO)(2) and Ni (eta (2)-NO)(2), differing by the metal coordination modes and electronic structures. For the M(eta (1)-NO)(2), M = Ni and Pd, species, four additional fundamentals have been measured in the mid- and far-infrared regions for various isotopic species. If the experimental results only show that the complex is centrosymmetrical, the existence of two IR-active vibrations in the far-infrared region and the data for the asymmetrical isotopomers are consistent with a linear structure predicted by DFT calculations. For the Ni(eta (2)-NO)(2) --> Ni(eta (1)-NO)(2) conversion, estimates of the transition state energetic and structural properties are consistent with the experimental data. The changes in electronic structure and the reinforcement of the coordination bond when going from mono- to dinitrosyls are discussed in light of the results of a topological analysis of the bonding.