The preparation and molecular structure of the initial nanosized platinum-gold carbonyl cluster, Pt-13[Au-2(PPh3)(2)](2)(CO)(10)(PPh3)(4) (1), are described. A comparative analysis reveals its pseudo-D-2h geometry, consisting of a centered Pt-13 icosahedron encapsulated by two centrosymmetrically related bidentate [Ph3PAu-AuPPh3]-capped ligands along with 4 PR3 and 10 CO ligands, to be remarkably similar to that of the previously reported Pt-17(mu(2)-CO)(4)-(CO)(8)(PEt3)(8) (2). Reformulation of 2 as Pt-13[(PtPEt3)(2)(mu(2)-CO)](2)-(CO)(10)(PEt3)(4) emphasizes the steric/electronic resemblance of the bulky-sized bidentate [Ph3PAu-AuPPh3] and [(PtPEt3)(2)(mu(2)-CO)] capping ligands in 1 and 2, respectively, as well as their identical electron counts of 162 cluster valence electrons for a centered Pt-13 icosahedron. We hypothesize that analogous steric effects of their ligand polyhedra in 1 and 2 play a crucial role along with electronic effects in the formation and stabilization of these two nanosized clusters that contain an otherwise unknown centered icosahedron of platinum atoms.