Photogeneration of Several Linkage Isomers and Investigation of Forward and Backward Nitro-Nitrito Isomerization Processes in a Palladium Complex.

Photoinduced linkage isomers (PLIs) of the nitro-ligand were generated and comprehensively characterized in a square planar unit [Pd(NH 3 ) 3 NO 2 ] + of the complex salts [Pd(NH 3 ) 4 ][Pd(NH 3 ) 3 NO 2 ][MOx 3 ]· y H 2 O (M = Cr ( Cr ), Rh ( Rh ), Co ( Co ), Ox = oxalate). Structural (XRD) and spectroscopic (IR, UV-vis) investigations at 10 and 150 K allowed determining the structures of several photoinduced linkage isomers, endo -ONO (PLI1, 2) and exo -ONO (PLI3, 4) isomers generated by irradiation with 365 nm from the initial NO 2 (GS), along with the assignment of the infrared (IR) bands to each structural isomer. Based on a combination of these methods, the photo- and thermally induced interplay of PLIs was investigated. Irradiation in the temperature range of 10-80 K induces the formation of both endo - and exo -ONO isomers, while increasing the temperature up to 150 K results in the formation of only endo -ONO isomers. The structural arrangement of the endo -ONO and exo -ONO PLI is strongly influenced by intermolecular interactions due to the partial occupation of a neighboring site by water molecules. The investigation of thermal dynamics of PLIs revealed that the thermal decay of the exo -ONO isomer occurs via two steps exo -ONO → endo -ONO → NO 2 . The kinetic parameters ( E a , k 0 ) of both decay processes were determined together with the characteristic decay temperatures ( T d ) by IR spectroscopy. According to the photoinduced dynamics measured by IR spectroscopy, the mechanism of PLI formation in [Pd(NH 3 ) 3 NO 2 ] + could be described as NO 2 → endo -ONO → exo -ONO.