Structural and bonding trends in platinum-carbon clusters

Density functional calculations with the B3-LYP functional were used to opt imize the platinum-carbon cationic clusters, PrCx+, 1 less than or equal to x less than or equal to 16, in both the doubler and quartet states of the linear, fan, opec-ring, closed-ring, and one-carbon-ring geometries. Trends in stability, Pt+-C-x binding energy, doublet-quartet excitation energy, a nd Pt-C bond lengths were investigated. Explanations for these patterns are provided in terms of orbital interactions and changes imposed on the carbo n chain by the metal atom. In accord with the previously studied palladium- carbon cations, the PtCx+ clusters favored a linear geometry for 3 less tha n or equal to x less than or equal to 9. For larger clusters, the open-ring (Pt inserted in C-x ring) and closed-ring (Pt bound to two atoms of closed C-x ring) families exhibit the lowest-energy structures. The stability and the nature of the Pt-C bonding in both the closed-ring and one-carbon-ring (Pt bound to one atom of closed C-x ring) PtCx+ structures depend greatly on the aromaticity of the corresponding C-x ring. However, unlike their pal ladium counterparts, the closed-ring platinum clusters were found invariabl y to be more stable than the respective one-carbon species. The stability o f forming two Pt--C sigma bonds is due to relatively lower energy sd hybrid orbitals from the platinum cation.