An Infrared Study of Gas-Phase Metal Nitrosyl Ion-Molecule Complexes.

We present a combined experimental and quantum chemical study of gas-phase group 9 metal nitrosyl complexes, M(NO) n + (M = Co, Rh, Ir). Experimental infrared photodissociation spectra of mass-selected ion-molecule complexes are presented in the region 1600 cm -1 to 2000 cm -1 which includes the NO stretch. These are interpreted by comparison with the simulated spectra of energetically low-lying structures calculated using density functional theory. A mix of linear and nonlinear ligand binding is observed, often within the same complex, and clear evidence of coordination shell closing is observed at n = 4 for Co(NO) n + and Ir(NO) n + . Calculations of Rh(NO) n + complexes suggest additional low-lying five-coordinate structures. In all cases, once a second coordination shell is occupied, new spectral features appear which are assigned to (NO) 2 dimer moieties. Further evidence of such motifs comes from differences in the spectra recorded in the dissociation channels corresponding to single and double ligand loss.