Reaction of [(eta-C(5)Me(5))Mo(O)(mu-O)](2) with [(eta-C(5)Me(5))Mo(CO)(2)]2 plus O-2 give (eta-C(5)Me(5))(4)Mo5O11. This cluster contains (eta-C(5)Me(5))Mo(O)(2) attached to a [(eta-C(5)Me(5))Mo(mu-O)](3)(mu(3)-O)(3)Mo(O)(2) unit-by a bridging oxygen atom. The [Mo(mu-O)](3)(mu(3)-O)(3)Mo structure is favored over the alternative adamantane-like [M(4)(mu-O)(6)] structure because of the pi-bonding of the terminal oxygen atoms attached to the apical Mb atom. The [(eta-C(5)Me(5))Mo(mu-O)](3)(mu(3)-O)(3)Mo(O)(2) unit contains four cluster electrons which are associated with the three basal molybdenum atoms. The magnetic moment of (eta-C(5)Me(5))(4)Mo4O11 was 3.16 mu(B) at 291 K, indicating that the cluster core has two unpaired electrons. Crystal data : C40H60Mo5O11, triclinic, P (1) over bar, a = 10.851(2) Angstrom, b = 12.450(4) Angstrom, c = 18.153(3) Angstrom, alpha = 91.07(2)degrees, beta = 102.04(1)degrees, gamma 109.27(2)degrees, Z 2, R = 0.063 for 4575 observed reflections and 505 parameters. Reduction of (eta-C(5)Me(5))MoCl(O)(2) with zinc in HCl/CHCl3 gave {[(eta-C(5)Me(5))Mo](3)(mu-OH)(n)(mu-O)(6-n)}Cl-2, which was characterized by elemental analysis, mass spectrometry, and IR, NMR, and EPR spectroscopies. A partial crystal structure analysis showed that the trinuclear cluster had oxygen or hydroxo ligands bridging the edges of a molybdenum triangle. The magnetic moment (1.13 mu(B) at 4 K) and NMR and EPR spectra suggested that the cluster with n = 5 (six cluster electrons, diamagnetic) was in a redox equilibrium with clusters with n = 4 (five cluster electrons, one unpaired electron) and n = 6 (seven cluster electrons, one unpaired electron). This is the first example of a redox equilibrium in a cluster of this type. The cation {[(eta-C(5)Me(5))Mo](3)(mu-OH)n(mu-O)(6-n)}(2+) is compared to related {[(eta-C(5)R(5))M](3)(mu-A)(n)(mu-O)(6-n)}(q+) clusters. Crystal data : C44H66Cl2Mo3O6, monoclinic, C2/c, a = 27.97(4) Angstrom, b = 11.260(6) Angstrom, c = 16.865(15) Angstrom, beta = 117.93(8)degrees, Z = 4, R = 0.147 for 921 observed reflections and 61 parameters.