Abstract A new classification scheme for transition metal carbonyl cluster compounds is proposed. Class I clusters are defined as those which use all available Cluster Valence Molecular Orbitals, CVMOs, Class II cluster compounds have s and p CVMOs vacant and Class III clusters have vacant d CVMOs. In general the earlier transition elements to the left of the transition metal series will have more of a tendency to form Class III cluster compounds, while the later transition elements to the right of the transition series will tend to form Class II compounds. The more common Class I cluster compounds will be favored by the central Group VIII metals such as Rh. Only the Class I clusters will follow the Polyhedral Skeletal Electron Pair analogy between main group and transition metal clusters and will exhibit the “magic numbers” of Cluster Valence Electrons, CVEs.