Reaction of Mo-2(pyphos)(4) (1) with [MCl(CO)(2)](2) (M = Ir and Rh) afforded linear tetranuclear complexes of a formula Mo2M2(CO)(2)(Cl)(2)(pyphos)(4) (2, M = Ir; 3, M = Rh). X-ray diffraction studies confirmed that two "MCl(CO)" fragments are introduced into both axial sites of the Mo-2 core in 1 and coordinated by two PPh2 groups in a trans fashion, thereby forming a square-planar geometry around each M(I) metal. Treatment of 2 and 3 with an excess amount of (BuNC)-Bu-t and XylNC induced dissociation of the carbonyl and chloride ligands to yield the corresponding dicationic complexes [Mo2M2(pyphos)(4)((BuNC)-Bu-t)(4)](Cl)(2) (5a, M = Ir; 6a, M = Rh) and [Mo2M2(pyphos)(4)(XylNC)(4)](Cl)(2) (7, M = Ir; 8, M = Rh). Their molecular structures were characterized by spectroscopic data as well as X-ray diffraction studies of BPh4 derivatives [Mo2M2(pyphos)(4)((BuNC)-Bu-t)(4)](BPh4)(2) (5b, M = Ir; 6c, M = Rh), which confirmed that there is no direct sigma-bonding interaction between the M(I) atom and the Mo-2 core. The M(I) atom in 5 and 6 can be oxidized by either 2 equiv of [Cp2Fe][PF6] or an equimolar amount of I-2 to afford Mo(II)(2)M(II)(2) complexes, [Mo2M2(X)(2)((BuNC)-Bu-t)(4)(pyphos)(4)](2+) in which two Mo-M(II) single bonds are formed and the bond order of the Mo-Mo moiety has been decreased to three. The Ir(I) complex 5a reacted not only with methyl iodide but also with dichloromethane to afford the 1,4-oxidative addition products [Mo2Ir2(CH3)(I)((BuNC)-Bu-t)(4)(pyphos)(4)](Cl)(2) (13) and [Mo2Ir2(CH2Cl)(Cl)((BuNC)-Bu-t)(4)(pyphos)(4)](Cl)(2) (15), respectively, although the corresponding reactions using the Rh(I) analogue 6 did not proceed. Kinetic analysis of the reaction with CH3I suggested that the 1,4-oxidative addition to the Ir(I) complex occurs in an S(N)2 reaction mechanism.