Distinctive Coordination of CO vs N₂ to Rhodium Cations: An Infrared and Computational Study

Carbonyl and nitrogen complexes with Rh⁺ are produced in a molecular beam using laser ablation and a pulsed-nozzle source. Mass-selected ions of the form Rh(CO)_n⁺ and Rh(N₂)_n⁺ are investigated via infrared laser photodissociation spectroscopy. The fragmentation patterns and infrared spectra provide information on the coordination and geometries of these complexes. The shifts in vibrational frequencies relative to the uncoordinated ligands give insight into the nature of the bonding interactions involved. Experimental band positions and intensities are compared to those predicted by density functional theory (DFT). Rh⁺ coordinates only four nitrogen molecules, whereas it can accommodate five carbonyl ligands. The fifth CO ligand resides in an axial site with bonding intermediate between coordination and solvation. The carbonyl stretch in Rh(CO)₄⁺ (2160 cm^-1) is blue-shifted with respect to the molecular CO vibration (2143 cm^-1). Conversely, the N-N stretch in Rh(N₂)₄⁺ (2297 cm^-1) is red-shifted with respect to the free N₂ vibration (2330 cm^-1). The opposite directions of these frequency shifts is explained by a combination of σ donation and electrostatic ligand polarization.