the center of the cluster. The remaining two unhybridized orbitals lie along the tangent of the sphere like structure creating more bonding and antibonding orbitals between the boron vertices. The orbital diagram breaks down as follows: The 18 framework molecular orbitals, (MOs), derived from the 18 boron atomic orbitals are: 1 bonding MO at the center of the cluster and 5 antibonding MOs from the 6 sp-radial hybrid orbitals 6 bonding MOs and 6 antibonding MOs from the 12 tangential p-orbitals. The total skeletal bonding orbitals is therefore 7, i.e. . Transition metal clusters Transition metal clusters use the d orbitals for bonding. Thus they have up to nine bonding orbitals, instead of only the four present in boron and main group clusters. Clusters with interstitial atoms Owing their large radii, transition metals generally form clusters that are larger than main group elements. One consequence of their increased size, these clusters often contain atoms at their centers. A prominent example is [Fe6C(CO)16]2-. In such cases, the rules of electron counting assume that the interstitial atom contributes all valence electrons to cluster bonding. In this way, [Fe6C(CO)16]2- is equivalent to [Fe6(CO)16]6- or [Fe6(CO)18]2-. References General references Chemical bonding Inorganic chemistry Organometallic chemistry Cluster chemistry