The flattened deltahedra and related polyhedra found in hypoelectronic bare group 13 metal cluster anions are also found in some anionic mixed rhodium-nickel carbonyl clusters. In all cases the rhodium vertices rather than the nickel vertices are involved in the flattening process so that the rhodium vertices contribute four internal orbitals and the nickel vertices three internal orbitals to the skeletal bonding of the cluster. Thus, the 11-vertex cluster Rh5Ni6(CO)(21)(3-) has a D-3h triflattened pentacapped trigonal prismatic structure similar to that found in the In-11(7-) anion of the intermetallic K8In11. Similarly the polyhedra in the 11-vertex cluster RhNi10(CO)(19)(3-) and the 9-vertex cluster Rh3Ni6(CO)(17)(3-) are both derived from a 10-vertex isocloso polyhedron by capping (for RhNi10(CO)(19)(3-)) or vertex removal (for Rh3Ni6(CO)(17)(3-)) followed by flattening all of the rhodium vertices. A D-3h icosahedron with flattened rhodium vertices is found in the 12-vertex cluster Rh3Ni9(CO)(22)(3-).