EXPLORATION OF THE IONIZABLE METAL CLUSTER ELECTRODE SURFACE ANALOGY - INFRARED SPECTROELECTROCHEMISTRY OF [PT24(CO)30]N, [PT26(CO)32]N, AND [PT38(CO)44]N (N = 0 TO -10) AND COMPARISONS WITH POTENTIAL-DEPENDENT SPECTRA OF CO ADLAYERS ON PLATINUM SURFACES

Infrared spectroelectrochemistry has been utilized to explore the vibrational properties of the high-nuclearity platinum carbonyl clusters [Pt24(CO)30]n, [Pt26(CO)32]n, and [Pt38(CO)44]n as a function of the charge n in dichloromethane, acetonitrile, acetone, tetrahydrofuran, and methanol. The clusters exhibit unusually reversible voltammetric and spectroelectrochemical behavior, with a sequence of redox steps spanning n = 0 to (in one case) -10, having formal potentials, E(f), between ca. 0.5 and -2.5 V vs ferrocenium-ferrocene. Largely two-electron steps are observed for [Pt26(CO)32]n, involving even-charge states (n = -2, -4, -6, -8, -10). Sequential one-electron steps are found for [Pt24(CO)30]n and [Pt38(CO)44]n, although the regions of electrode potential over which odd-charge states (n = -1, -3, -5, -7) are stable (i.e., the spacings between E(f) values) are markedly smaller than those for the even-charge states. The C-O stretching frequencies for the bridging (nu(CO)b) and especially the terminal (nu(CO)t) coordinated CO ligands decrease systematically as n becomes more negative; for example, nu(CO)t for [Pt24(CO)30]n diminishes by 15-20 cm-1 per added electron. The observation of such remarkable charge-dependent spectral properties for these large and structurally well-defined platinum clusters invites comparisons with the potential (and consequent charge)-dependent properties of CO adlayers at corresponding platinum electrode-solution interfaces. The latter systems also display decreases in nu(CO)t and nu(CO)b as the electrode potential, E, and hence the surface charge is made more negative, as usually ascribed either to increased d-pi(Pt) --> pi*(CO) backbonding or to a Stark effect. The nu(CO)t-E slopes for saturated CO adlayers at both single-crystal and polycrystalline Pt-nonaqueous interfaces are noticeably smaller than for the corresponding solvated Pt carbonyl clusters, the latter being adjudged from the nu(CO)t-E(f) behavior. These differences are due chiefly to larger "effective surface" capacitances (i.e., charge-E(f) dependencies) for the clusters than those measured for the electrode-solution interfaces. Such differing capacitances can largely be accounted for by a simple geometric electrostatic model. When the nu(CO)t values are plotted versus the electronic charge per surface Pt atom ("surface charge density"), however, an essentially uniform nu(CO)t-charge dependence is observed for the different Pt clusters, with similar behavior being obtained for the Pt electrodes. These comparisons provide an intriguing link between the electronic and bonding properties of such large ionizable metal clusters with those of chargeable metal surfaces.