Using the scanning tunneling microscope (STM) at 5 K, we have observed spontaneous formation of clusters between CO molecules adsorbed on the Au(111) surface. In all the (CO)(n) clusters (n = 2-5), the CO molecules are bonded to the ilea rest-neighbor gold atoms in characteristic arrangements. The CO dimer was found to exhibit an unusual tip-induced motion, where one molecule orbits around its neighbor. The clusters could be translated and manipulated without decomposition using the STM tip. These results demonstrate that the interaction between CO molecules bonded in particular cluster geometries to the nearest-neighbor An atoms is attractive rather than repulsive as commonly assumed. (c) 2006 Elsevier B.V. All rights reserved.