A number of zirconium iodide clusters that contain an interstitial atom in the center of a distorted octahedron of zirconium atoms have been synthesized in high yield and structurally characterized. Two distinct types of clusters have been found with stoichiometries Zr(,6)I(,12)X and MZr(,6)I(,14)X (where X = interstitial atom and M = Cs, Rb, or K). The two types of phases both contain Zr(,6)I(,12)X units but differ in the manner in which these units are connected by the sharing of iodines terminal to each zirconium;The variety of interstitial atoms found in the centers of these clusters is remarkable and includes carbon in Zr(,6)I(,14)C, MZr(,6)I(,14)C, and Zr(,6)I(,12)C; boron in MZr(,6)I(,14)B and Zr(,6)I(,12)B; silicon in Cs(,0.3)Zr(,6)I(,14)Si; aluminum in Cs(,0.7)Zr(,6)I(,14)Al; and potassium in Zr(,6)I(,14)K(,x) (x = 1 and 0.5). The phases represent the first example of an alkali metal atom, silicon, or aluminum occupying the center of a transition metal halide cluster;The structures of nine of these compounds have been determined by single crystal X-ray diffraction studies. The results of these studies demonstrate that while the cell volumes of the MZr(,6)I(,14)X vary by only 3.5% the Zr-Zr bond distances differ by nearly 8.5%. The reason for this difference in trends involves the large 'matrix effect' observed in zirconium iodide clusters. A simple geometric model describing the observed changes in cluster geometry is proposed;Extended Huckel calculations on nonmetal-centered clusters demonstrate that the valence s and p orbitals of the interstitial atom interact with four orbitals primarily involved in Zr-Zr bonding to give four low-lying Zr-interstitial bonding orbitals. Molecular orbital calculations on the potassium-centered cluster indicate that stability is gained by a combination of Zr-I and much smaller K-cluster interactions that offset the loss of Zr-Zr bonding upon expansion of the cluster to accommodate the potassium. Extended Huckel;calculations also demonstrate the importance of the six terminal iodides on the symmetry and Zr-Zr bonding in the clusters; ('1)DOE Report IS-T-1164. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.