CHARACTERIZATION OF HIGH-NUCLEARITY CLOSE-PACKED ANIONIC PLATINUM CARBONYL CLUSTERS BY CF-252 PLASMA DESORPTION MASS-SPECTROMETRY

Cf-252-plasma desorption mass spectrometry has been used to investigate a series of high nuclearity, di- and tetraanionic platinum carbonyl clusters containing Pt19, Pt24, Pt26 and Pt38 closest-packed metal cores. Abundant singly-charged negative ions produced by loss of an electron, form an envelope of peaks corresponding to successive losses of carbonyl ligands from the intact metal core. A remarkable series of oligomeric negative ions formed by self-condensation of the metal core are also observed for the dianionic clusters that extend beyond m/z 50 000. Slightly less abundant positive parent and oligomer ions are also observed for these clusters. The oligomer peaks extend beyond m/z 100 000 in the spectrum of the Pt26 ClUster. The astonishing formation of these positively charged aggregates that contain in excess of 500 Pt atoms is even more remarkable because there is no incorporation of the associated cation despite the strong presence of the cation in the positive ion spectrum. These investigations have established that Cf-252-PDMS can be used to unequivocally identify the platinum stoichiometry of these high nuclearity clusters, thus providing a new tool in the vast array of spectroscopic techniques used to structurally characterize solution-soluble metal clusters. The unusual ions formed by this class of compounds represent the largest positive and negative ions observed by Cf-252-PDMS. They may also provide a probe of the complex reactions that occur in the fission fragment track and in the ejected plume of neutrals and ions. These large Pt clusters give quite good intensities even at m/z > 50000 where the mechanism for detection must involve potential electron emission. The electronic structure of these metal clusters may be revealing a new aspect of potential emission where the Fermi level structure of the incident ion plays a role.