The additional occupation of the face centers of the M, octahedron of a M,X,, cluster is observed for the first time in Th,Br,,H,; this type of occupation is already known for systems with condensed clusters. In the compounds ZrXH (X = C1, B I -) [ '~~ and LnXH,[I5] two H atoms assume positions in opposite faces of elongated trigonal antiprisms of M atoms (d(D-D) = 220 pm in the case of ZrBrD and 298 pm in the case of TbBrD,).The number of bonding orbitals in the Th,Br,,H, cluster is the same as in the hypothetical empty Th,Br,, cluster: bonding H-Th interactions merely lead to a lowering of individual orbital energies; the antibonding combinations lie above the HOMO-LUMO gap. Thus, the electron balance for Th,Br,,H, (6 x 4 -15 + 7 = 16) corresponds exactly to that for a complete occupation of all bonding cluster orbitals (for comparison Nb,F,, : 6 x 5 -15 = 15). The metal-metal bonding in the (empty) M,X,, cluster is best described in terms of closed three-center two-electron bonds in all eight faces of the M, octahedron['61 and provides a clear picture of the bonding contribution of the H atoms. Seven of these multicenter bonds are reinforced by the mixing of the 1 s function of the respective H atom in the center of the bond and by the additional electron of each H atom, so that finally there are seven four-center two-electron bonds and one three-center two-electron bond.Since the electron balance already varies in the case of empty clusters (e.g. Ta,Cl;Y with n = 2-4, corresponding to 16 to 14 electrons in M-M bonding states['71) and can be further decreased in clusters with interstitial atoms (e.g. Sc,CI,,B corresponding to 12 electrons'181), it is to be expected that the hydrogen content in Th,Br,,H, is variable. Further investigations are currently under way. Received: September 3, 1990 [Z 4165 IE] German version: Angew. Chem. 103 (1991) 79 CAS Registry numbers: Th,Br,,H,, 131 104-1 6-0; Th,Br,,D,, 1311 30-13-7.