The adsorption geometry and the nature of the interaction of the SCN molecule at an on-top site of the Ag(100) surface have been investigated using ab initio cluster model wave functions. The SCN anion, SCN-, is a bidentate ligand. If the SCN-Ag bond is ionic, we could expect, by analogy with thiocynate-metal complexes, that SCN could be bound to a metal surface through either the N end or the S end. We show that the chemisorption bond for SCN/Ag is very ionic and that the interaction between chemisorbed SCN and the Ag surface is largely electrostatic. The most important bonding mechanism is the polarization of the Ag surface due to the presence of SCN-. However, we do find that there is a small, but non-negligible, covalent interaction. There is a very small energetic cost to change the angle of SCN with respect to the surface between a perpendicular and a parallel orientation. We contrast this with the case of a covalently bonded adsorbate, CO/Ag(100), where the π bond strongly favors orientations near perpendicular. The flat potential energy curve for bending SCN suggests that the adsorption geometry at high SCN coverage may be largely determined by nonbonding interactions between adjacent adsorbates. © 1991 American Institute of Physics.