Two metal carbonyl cluster octahedra can be fused by sharing a vertex to give an Mrr cluster, by sharing an edge to give an Ml,, cluster, or by sharing a face to give an Mg cluster. In a globally delocalized metal carbonyl cluster consistmg of fused octahedra, vertrces unique to a single octahedron use three internal orbrtals, vertices of a face shared by two octahedra use four internal orbitals, vertices of an edge shared by two octahedra use five internal orbitals, and vertices of a face shared by two octahedra use six Internal orbitals. Using this prmciple the iridium carbonyl clusters Irg(CO)2,,-and Ir12(CO)262-, constructed by face-sharing fusion of two and three octahedra, respectively, are seen to be electron-and orbital-precise and thus may be regarded as three-dimensional analogues of naphthalene and anthracene, respectively. These iridium carbonyl clusters are the second and third members of a homologous series Ir3+3n(C0)(2, + ,,+; the end member of this series 1s the polymer [Ir,(CO),,], consistmg of an infinite cham of octahedra sharing opposite faces. Other electron-and orbital-precise clusters constructed by the face-sharmg fusion of two octahedra are the mixed metal derivatrve Ir,Nr,(C0),,3-and the Ni, core in nickel clusters Ni12(C0)21H4_-n"-; in the latter clusters the three symmetry-related edges of the central Nig core are bridged by Ni(C0)2 groups. The rhodium carbonyl clusters Rhg(C0)r9'-and H2RhlZ(C0)25 both have two skeletal electrons less than then Iridium carbonyl counterparts Ir,(C0)aa3-and Ir,2(C0)262-, respectrvely, and can both be regarded as mrssing a pair of core bonding electrons; this arises by interaction between the symmetric S" core molecular orbitals in adjacent face-fused octahedra in the rhodium clusters which rarses the energy of one these orbrtals to antibonding levels. The gold-nickel 'Spiro' mixed cluster Au,NI,,(CO),*-consrsts of four N&Au, octahedra which are fused by sharing each of the six gold vertices between two octahedra leading to overall Td symmetry, this cluster as well as Ru,,,~(CO)~~~-consisting of two edge-sharing octahedra are electron-and orbital-precise.