Constrained Molecular Manipulation Mediated by Attractive and Repulsive Tip-Adsorbate Forces**

The forces between the tip of a scanning tunneling microscope and an adsorbed molecule may be exploited to induce a range of translational and conformational modes of manipulation. [1–12] As shown by Bartels et. al., [2] different modes of translational manipulation may be distinguished through the acquisition of the trajectory executed by the scanning tunneling microscopy (STM) instrument tip as the adsorbate moves across the surface. In most cases the tip height changes abruptly as the molecule is translated between discrete adsorption sites on the surface, enabling a distinction between attractive and repulsive modes of manipulation. The abrupt change occurs when the molecule hops, since there is an effective change in the tip–surface separation, which results, when operating under constant-current mode, in an adjustment of tip height. An interesting variant, which occurs for weakly bound adsorbates under low-temperature conditions, is a sliding mode, in which discontinuous changes in tip height are absent and the adsorbate moves across the surface in a quasi-continuous manner rather than hopping between discrete adsorption sites. [2] In this Communication, we describe a mode of manipulation in which a balance between attractive and repulsive tip–molecule forces results in the stabilization of a molecule at a series of intermediate positions between the adsorption sites adopted by a free molecule on the surface. We find that the tip induces a type of repulsive manipulation, in which abrupt molecular hops are suppressed through a residual attractive interaction with the tip. There are some similarities between the acquired STM line scans and those previously categorized as sliding in low-temperature experiments. [2] However, for the large, strongly bound,