Influence of interstitial atoms in transition-metal carbonyl clusters

The interactions between interstitial atoms (i.a.s) and cluster orbitals have been classified using qualitative perturbation theory arguments. The number of cluster valence orbitals contributed by the i.a. is shown to depend on the energy, shape and contraction of the i.a. and cage orbitals. Stereochemical effects of the i.a.s on transition-metal carbonyl clusters are discussed on the basis of molecular mechanics and extended Hückel computations. In general, a strong influence of the interstitial atoms on the ligand stereogeometry is expected. However, the energy and the shape of the cluster orbitals depend on the details of the ligand stereochemistry and the exact relationships between the inner interstitial atom, the cluster stoichiometry, and the outer ligand geometry need to be determined for each individual case. Interstitial atoms can affect both the cluster electronic requirements and the ligand stereochemistry and cannot merely be considered as innocent 'internal ligands'.