ISOTOPIC LIGAND-EXCHANGE IN THE HIGH-NUCLEARITY PLATINUM CLUSTERS [PT24(CO)30]N AND [PT26(CO)32]N (N = 0 TO -6) IN DICHLOROMETHANE - SUBSTITUTION KINETICS, CARBONYL DIPOLE COUPLING, AND COMPARISONS WITH PT(111) ELECTRODES

The effect of substituting (CO)-C-12 by (CO)-C-13 on the infrared spectra of the electroactive [Pt24(CO)30]n and [Pt26(CO)32]n series (n = 0 to -6) in dichloromethane has been studied in order to explore the nature of the ligand substitution kinetics and dynamic dipole-dipole coupling for these high-nuclearity metal clusters in relation to metal surfaces. The chargeable nature of the platinum clusters, as scrutinized by FTIR spectroelectrochemical tactics, offers interesting comparisons with the potential-dependent dipole coupling observed for mixed (CO/CO)-C-13-C-12 adlayers at the Pt(111)-dichloromethane interface, also reported here. Unlike the electrochemical surface, CO ligand exchange on [Pt24(CO)30]2- and [Pt26(CO)32]2- is slow and nonrandom, incomplete (CO/CO)-C-12-C-13 substitution occurring even on long (more than several hours) time scales. Substantial deviations from first-order substitution kinetics are also observed, especially for [Pt24(CO)30]2- Where only 65% of the CO ligands are observed to undergo exchange. The results suggest the occurrence of a "merry-go-round" ligand-migration mechanism from active substitution centers within the monocrystalline Pt facets that make up the cluster surfaces. Consistent with this, the formation of high local (CO)-C-13 coverages even early in the substitution process is indicated from the strong (CO)-C-13 dipole coupling, deduced from the terminal C-O stretching frequencies, nu(CO)t, together with the nonsymmetrical nature of the 13-nu(CO-)t and 12-nu(CO-)t isotope mole fraction plots. Even though only small CO islands can be formed on the clusters, the nu(CO)t upshifts due to dynamic dipole-dipole coupling, DELTA-nu(D), approach those observed, 30-35 cm-1, for the Pt(111)/CO interface. The magnitude of the dipole-dipole coupling is also dependent on the surface electronic charge for both the clusters and the electrode surface, yet in different ways. For the letter, DELTA-nu(D) diminishes markedly as the electrode charge becomes more negative.