Molybdenum Complexes of C,C-Bis(ethynyl)carboranes: Design, Synthesis, and Study of a Weakly Coupled Mixed-Valence Compound

The design and study of organometallic mixed-valence complexes is complicated by the mixing of metal d and bridging ligand pi orbitals, which often makes the assignment of metal oxidation states ambiguous. However, in the case of complexes based on the cycloheptatrienyl-ligated molybdenum fragment, Mo(dppe)(eta-C7H7), the strong ring to metal pi bonding and metal to ring 6 back-bonding interactions stabilize four of the metal d orbitals, while the d(z2) orbital is destabilized by filled-filled interactions with the a-type MO of the C7H7 ring. When the Mo(dppe)(eta-C7H7) auxiliary is used in conjunction with a 1,12-bis(ethynyl)-1,12-carbaborane-based bridging ligand, a weakly coupled (Robin and Day class II) mixed-valence system, [{Mo(dppe)(eta-C7H7)}(2){mu-1,12-(C equivalent to C)(2)-1,12-C2B10H10}(+) ([2](+)), with well-defined molybdenum oxidation states can be prepared. The near-IR region of [2](+) exhibits three intervalence charge transfer (IVCT) transitions and two lower intensity interconfigurational (or dd) transitions, which have been resolved through spectral deconvolution. The band shape of the lowest energy IVCT transition associated with [2](+), which arises from electron exchange between the d(z2)-type orbitals at the two Mo centers, is in excellent agreement with the predictions of the Hush two-state model for weakly coupled mixed-valence complexes. The half-height bandwidths of the higher energy IVCT transitions, which arise from transitions between lower lying metal orbitals that have symmetry properties that permit more significant mixing with the bridging ligand, are in less good agreement with the Hush model, due to the breakdown of the two-state approximation through the greater involvement of the bridge-based orbitals in those transitions.