The reaction of Ir-4(CO)(12) with an excess of Pt(PBu3)(2) at room temperature yielded the bis-Pt(PBu3t) adduct Ir-4(CO)(12)[Pt(PBu3)](2) (9), which contains two Pt(PBu3t) groups bridging opposite edges of a central Ir-4 pseuclotetrahedron. The same reaction at 110 degreesC yielded two new higher nuclearity complexes, Ir-8(CO)(12)[Pt(PBu3t)](4) (10) and Ir-6(CO)(10)[Pt(PBu3t)](4) (11). Compound 10 consists of a central Ir-4(CO)(4) tetrahedron with four edge-bridging Ir(CO)(2) groups and four Pt(PBu3t) groups that are each bonded to Ir-3 triangles of the Ir-4 tetrahedron and two of the Ir(CO)(2) groups. Compound 11 consists of a central Ir-4(CO)(4) pseuclotetrahedron with two edge-bridging Ir(CO)(2) groups and four Pt(PBu3t) groups; one Pt(PBu'3) group is bonded to five iridium atoms as found in 10; two are bonded to four iridium atoms, and one is bonded to one of the outer Ir2Pt triangles. Compound 11 reacted with hydrogen at 97 degreesC to give the new tetrahydrido complex Ir-6(CO)(8)[Pt(PBu3t)](4)(mu-H)(4) (12). Compound 12 is formed by the loss of the two bridging carbonyl ligands from 11 and the addition of four hydrido ligands. All four new compounds were characterized by both H-1 and P-31 NMR and by single-crystal X-ray diffraction analyses. The bonding in 9 was studied by Fenske-Hall molecular orbital calculations, which in this case provides a delocalized bonding description for the Ir-Ir and Ir-Pt bonding, where the attachment of the 0 e(-) fragments of Pt(PR3) use Ir-Ir bonding orbitals of the Ir-4(CO)(12) cluster to form multicenter Pt-Ir bonds.