The study of the physicochemical properties of high-nuclearity clusters consisting of several metal atoms is of considerable interest. Fundamental questions concerning the changes in physical and chemical properties as the number of metal atoms increases need to be resolved. In this paper we report studies of the electronic spectra of several high-nuclearity osmium cluster carbonyls. These are compounds in which polyhedral arrangements of metal atoms are surrounded by a shell or sheath of CO ligands.1 Solution visible/UV spectra for a set of high-nuclearity osmium carbonyls are shown in Figures 1–4, and a schematic energy level diagram for such clusters, adapted from ref 1, is shown in Figure 5. Some physical properties of Os6(CO)18were reported earlier,2 and the solution spectra of the clusters fPt3(CO)6]„2“ (n = 2–6) (Figure 6) have been discussed by Chang and Woolley.3 It is convenient to begin by reviewing these findings before discussing the spectra shown in Figures 1–4. Gas-phase photoemission studies of Os6(CO)18show that the ionization threshold (8 eV) occurs in the metal cluster d band, which extends down to about 12 eV; as usual in these systems, the carbonyl “5cr” and “lx” features overlap and peak at 14.5 eV (±1.5 eV).4 Transmission and optical spectroscopy suggest that this compound is a semiconductor with a band gap of eV, l in agreement with the resistivity measurements. The First strong electronic transitions peak at 2.7 eV (±0.2 eV) in both crystals and solution.2 The platinum cluster carbonyl dianions [Pt3-(CO)6]„2-(see Figure 6, for example) all show strong optical absorptions in the visible range. All of them are colored with the energy of the first intense absorption maximum in the visible, decreasing steadily as the cluster size increases (from 3.3 eV for n = 1 to 1.6 eV for n = 6). Early calculations by Chang and Woolley3 have been improved by Bullett,5 who also extended them to the Pt9 species (n = 3). Bullett's results give a significant improvement to the description of the levels at the HOMO-LUMO gap and suggest that the HOMO of the dianion is an in-phase combination of carbonyl 2tt* levels split off from the main group of 2tt* levels, which remain empty, as suggested by Lauher.6 This unusual feature arises from the particular geometry of these platinum carbonyls, which is based on the stacking of nearly planar layers of Pt3(CO)6moieties (Figure 6). The normal energy level pattern for cluster carbonyls is shown schematically in Figure 5; the metal cluster d levels lie in the gap between the M-CO bonding.