A number of main group element (E)-transition metal (M) cluster compunds based upon a hexacapped M-8 cube have been reported in two configurations : empty and with interstitial metal atoms in the center of the cube. Related molecules arise upon slow oxidation of the reaction product that forms when [Bi2CO4(CO)(11)](-) is treated with Mo(CO)(3)(toluene) giving Bi-Co clusters based upon tetragonally distorted tetracapped cubic metal arrays with interstitial metal atoms. These compounds formulated as [PPN](2)[Bi4Co9(CO)(16)]. 2THF ([PPN](2)[1]. 2THF) and [PPN](2)[Bi8Co14(CO)(20)]. 1.08THF([PPN](2)[2]. 1.08THF) have been characterized via single-crystal X-ray diffraction. An alternative description ascribes the metal cores to close-packed arrays based upon the cuboctahedron. The complexes are electron rich via conventional electron-counting formalisms. To understand these molecules more fully, a thorough theoretical analysis of compounds [1](2-) and [2](2-) was undertaken. The calculations indicate that the bonding of the interstitial metal atom with the metallic host somewhat differs from that found in their related metal-centered cubic species M-9(mu(4)-E)(6)L-8, due to stretching along the 4-fold axis. The HOMO-LUMO region is slightly antibonding between the Co-i atoms (i = interstitial) and the surrounding Bi and Co-p (p = peripheral) atoms. As with other members of this cubic cluster class, no HOMO-LUMO gaps are observed. The actual electron counts for [1]2(-) and [2](2-), however, favor stronger Co-p-Co-i and Bi-Co-i bonding than would be observed for the allowed higher electron counts. Crystallographic data for [PPN](2)[1]. 2THF : triclinic space group P (1) over bar; a = 13.560(5), b = 15.893(5), c = 11.884(4) Angstrom; alpha = 105.26(2), beta = 98.25(3), gamma = 83.25(3)degrees; Z = 1. Crystallographic data for [PPN](2)[2]. 1.08THF : triclinic space group P (1) over bar; a = 17.445(5), b = 18.234(3), c =9.033(3) Angstrom; alpha = 92.74(2), beta = 102.59(3), gamma = 78.52(2)degrees; Z = 1.