Chemisorption of carbon monoxide on platinum cluster anions

Reactions of platinum cluster anions with carbon monoxide are investigated using a flow tube reactor (flowing afterglow) instrument. The platinum cluster anions are prepared with a cathodic discharge source. The cluster anions, Pt n−(n≤7), react with carbon monoxide in a flowing helium/argon buffer gas at 0.5 Torr total pressure and at 300 K. Clusters with four or more platinum metal atoms react by addition of CO with an effective bimolecular reaction rate of greater than 70% of the calculated collision rate, while for n=3 the reaction rate is 16% of the collision rate. At high CO concentrations, additional CO molecules attach to the clusters to form platinum carbonyl anion intermediates, with little loss of metal atoms. Saturation limits were found, above which no further CO addition occurred. The saturated cluster species are Pt3(CO)6−, Pt4(CO)8−, Pt5(CO)9−, and Pt6(CO)10−. The application of simple geometry arguments and electron counting rules to infer possible skeletal structures of the metal clusters is discussed.