Chemisorption-Induced 2D–3D–2D Structural Transitions in Gold Heptamer: (CO)_nAu₇^– (n = 1–4)

CO chemisorption onto the Au₇^– cluster is investigated using photoelectron spectroscopy (PES) and ab initio calculations. It is found that CO binding can induce previously unreported 2D–3D–2D structural changes. The gold motif in the most stable structure of COAu₇^– is an intermediate between the two known stable 2D isomers of Au₇^–. Two minor isomers are observed in the PES of (CO)₂Au₇^–; one is due to an unprecedented 3D Au₇^– species with C_s symmetry. This 3D C_s Au₇ motif becomes a major isomer in (CO)₃Au₇^–. The most stable isomers of COAu₇^– and (CO)₂Au₇^– are planar with identical Au₇ motifs; the stable planar isomers of (CO)₃Au₇^– include not only the global-minimum structure of Au₇^– but also a planar hexagonal Au₇ motif. The PES spectrum of (CO)₄Au₇^– is markedly different, and its most stable structure consists of the global-minimum structure of Au₇^–, three terminal CO, and one bridging CO.