In this work, the thermolysis of (R(,4)N)Mo(CO)(,4)I(,3), where R = Pr, Bu was examined. Depending on the reaction conditions, one of the following tetranuclear cluster anions was obtained: Mo(,4)I(,11)('2-), Mo(,4)I(,11)('2.5-), or "Mo(,4)I(,10)('2-)". The Mo(,4)I(,11)('2-) anion is best made by the reaction between a stoichiometric quantity of iodine and (R(,4)N)Mo(CO)(,4)I(,3) in refluxing chlorobenzene. The x-ray crystal structure was determined for the black tetrabutylammonium salt, (TBA)(,2)Mo(,4)I(,11). The structure of the Mo(,4)I(,11)('2-) anion is best viewed as a fragment of the (Mo(,6)I(,8))I(,6)('2-) anion with metal atoms removed from cis cube faces. The resulting M(,4) cluster has a distorted tetrahedral (butterfly) structure. There are five strong Mo-Mo bonds ((')d(Mo-Mo) = 2.567 (ANGSTROM)) and a "long bond" between the tip metal atoms (d = 3.035 (ANGSTROM));A rough MO scheme was devised to explain the M-M bonding in the anion. This scheme predicted 6 bonding, 2 nonbonding, and 8 antibonding orbitals. Subsequent EHMO calculations performed by Miessner and Korolkov 1 showed that the covalent nature of the Mo-I bonding in the Mo(,4)I(,11)('2-) anion has significant effects for the total bonding of the cluster. Their calculations predict 8 MOs with substantial M-M AO character, and a slightly antibonding HOMO. The calculations also showed that the "long Mo-Mo bond" was due to a minimum in total bonding energy at a dihedral angle of 88.6(DEGREES) and not to any M-M bonding;The complete thermolysis of (TBA)Mo(CO)(,4)I(,3) at 80(DEGREES)C, either in refluxing benzene or under a dynamic vacuum, yields (TBA)(,5)(Mo(,4)I(,11))(,2). This salt can also be made by the zinc reduction of (TBA)(,2)Mo(,4)I(,11) at room temperature in acetonitrile. This material is assumed to be a mixed salt containing both the Mo(,4)I(,11)('2-) and Mo(,4)I(,11)('3-) anions;"(TPA)(,2)Mo(,4)I(,10)" is synthesized by bubbling hydrogen or nitrogen gas through a (TPA)Mo(CO)(,4)I(,3) thermolysis reaction. The (TBA) salt can be produced by either the zinc reduction of (TBA)(,2)Mo(,4)I(,11) in refluxing acetonitrile or by bubbling hydrogen gas through a (TBA)Mo(CO)(,4)I(,3) thermolysis reaction. It was tentatively concluded;that these salts contain two anions, Mo(,4)I(,10)('2-) and Mo(,4)I(,10)H('2-), based on analytical, spectral, magnetic, and electrochemical data; ('1)DOE Report IS-T-1138. This work was performed under Contract No. W-7405-eng-82 with the U.S. Department of Oil, Gas, and Energy;Reference;1 Miessner, H. and Korolkov, D. V. Z. Anorg. Allg. Chem. 1983, 496, 175.