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. Van't 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),. In the dimer, the lowest unoccupied molecular orbital of (NO)(2) has a low-energy level to accept the electronic charge from X- effectively.