Dimer formation of NO ligands in the gas-phase halide ion clusters X−(NO)n enhanced by a frontier orbital interaction

The gas-phase equilibria for the clustering reaction of halide ions(X−) with NO were measured with a pulsed electron-beam high-pressure mass spectrometer. Vanʹt Hoff plots of equilibrium constants led to the determination of the thermochemical stabilities for X−(NO)n with n=1 and n=2. The bond energies of X−(NO)2 (X−=Cl− and I−) are found to be about 1.5 times larger than those of X−(NO)1. This anomaly is due to the electron-spin pairing effect, i.e., dimer pair formation of the NO ligands in the cluster ions X−(NO)2. In the dimer, the lowest unoccupied molecular orbital of (NO)2 has a low-energy level to accept the electronic charge from X− effectively.