The molecular structures of a series of tetrahedral clusters containing the central [M-4(CO)(12)] unit have been examined in the light of the ligand polyhedral model. Within this series fall the three anionic clusters [Rh3Ru(CO)(12)](-), [Rh3Os(CO)(12)](-) and [Rh2Ru2(CO)(12)](2-). These are unusual in that the carbonyl polyhedron does not conform to the icosahedron {12} (symmetry 1), the cubeoctahedron or the anti-cubeoctahedron usually observed but instead are based on the nido-capped icosahedron {13-1}. The complete series exhibits a transformation from icosahedron (C-3v molecular structure adopted for [CO4(CO)(12)]) to the nido-capped icosahedron (C-s molecular symmetry adopted for [Rh3Os(CO)(12)](-)) according to the approach adopted by Dunitz and Burgi. These observations represent an extension of the LPM along the lines previously reported in our consideration of the polyhedral growth sequence in which each successive polyhedron with vertices from 4 to 12 is generated first by edge-cleavage and then a capping operation. However, it is clear that the presence of amionic charge and/or the presence of counter cation(s) plays a dominant role in the final adopted carbonyl arrangement and polyhedral form. (C) 2003 Elsevier Science B.V. All rights reserved.