Structural and bonding trends in platinum-carbon clusters

Density functional calculations with the B3-LYP functional were used to optimize the platinum−carbon cationic clusters, PtC_x^+, 1 ≤ x ≤ 16, in both the doublet and quartet states of the linear, fan, open-ring, closed-ring, and one-carbon-ring geometries. Trends in stability, Pt^+−C_x binding energy, doublet-quartet excitation energy, and Pt−C bond lengths were investigated. Explanations for these patterns are provided in terms of orbital interactions and changes imposed on the carbon chain by the metal atom. In accord with the previously studied palladium−carbon cations, the PtC_x^+ clusters favored a linear geometry for 3 ≤ x ≤ 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) PtC_x^+ structures depend greatly on the aromaticity of the corresponding C_x ring. However, unlike their palladium counterparts, the closed-ring platinum clusters were found invariably to be more stable than the respective one-carbon species. The stability of forming two Pt−C σ bonds is due to relatively lower energy sd hybrid orbitals from the platinum cation.