Reactions of platinum cluster anions with carbon monoxide are investig
ated using a flow tube reactor (flowing afterglow) instrument. The pla
tinum cluster anions are prepared with a cathodic discharge source. Th
e cluster anions, Pt(n)-(n less-than-or-equal-to 7), react with carbon
monoxide in a flowing helium/argon buffer gas at 0.5 Torr total press
ure and at 3(K) K. Clusters with four or more platinum metal atoms rea
ct by addition of CO with an effective bimolecular reaction rate of gr
eater than 70% of the calculated collision rate, while for n = 3 the r
eaction rate is 16% of the collision rate. At high CO concentrations,
additional CO molecules attach to the clusters to form platinum carbon
yl anion intermediates, with little loss of metal atoms. Saturation li
mits were found, above which no further CO addition occurred. The satu
rated cluster species are Pt3(CO)6-, Pt4(CO)8-, Pt5(CO)9-, and Pt6(CO)
10-. The application of simple geometry arguments and electron countin
g rules to infer possible skeletal structures of the metal clusters is
discussed.