Gas-phase clustering reactions of halide ions with carbonyl sulfide were studied using a pulsed electron-beam mass spectrometer. For F-, formation of a stable FCSO- species was found with the F--CSO binding energy of >30 kcal/mol (the calculated value = 33.41 kcal/mol). DFT calculations showed that the three-fold in-plane coordination to the n = I species is stable. For Cl-, Br- and I-, the bond energies of cluster ions are in the range of 5-10 kcal/mol with symmetric geometries. All cluster geometries except for F- with n = I are controlled by the sigma* vacant orbital of O=C=S with participation of the sulfur 3d(z2-(x2+y2)) atomic orbital. (C) 2004 Elsevier B.V. All rights reserved.