Unusual chemical reactivity in the reactions of Re2(CO)8(μ-H)(μ-η1,η2-CHCHBu) with 2,3-bis(diphenylphosphino)maleic anhydride (bma) and Re2(CO)8(bma) with Ni(cod)2: X-ray diffraction structures of Re2(CO)8(bma), zwitterionic Re(CO)4[Re(CO)4(bma)], and th

The reaction between Re2(CO)8(μ-H)(ν-η1,η2-CHCHBu) (1) and the diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) proceeds slowly at room temperature to furnish the expected dirhenium compound Re2(CO)8(bma) (2) and the zwitterionic compound Re(CO)4[Re(CO)4(bma)] (3). Both 2 and 3 have been isolated and characterized in solution by IR and 31P NMR spectroscopy, in addition to X-ray crystallography. The solid-state structure of 2 reveals a bridging bma ligand that spans the Re–Re vector and whose Re(CO)4 moieties exhibit a staggered rotational geometry. 3 consists of a Re(CO)4 unit that is coordinated to the bma ligand in a chelating fashion by the two PPh2 moieties, with the other Re(CO)4 unit being π-bound to the alkene bond of the bma ligand. Thermolysis of Re2(CO)8(bma) in the presence of added Ni(cod)2 affords the phosphido-bridged complex Re2(CO)8[μ-η1,η1-(O)](μ-PPh2) (4), whose identity has been established by solution spectroscopic methods and X-ray diffraction analysis. The redox properties of Re2(CO)8(bma) have been examined in detailed by cyclic voltammetry and constant-potential coulometry, where two fully reversible, one-electron reductions (0/1− and 1−/2−) were observed at E1/2=−0.38 V and E1/2=−1.29 V in CH2Cl2. The reduction behavior is discussed relative to the nature of the LUMO level, which has been determined by extended Hückel MO calculations. The redox chemistry and the LUMO in Re2(CO)8(bma) are contrasted with the known redox chemistry and the LUMO composition of the mononuclear species BrRe(CO)3(bma) and related bma-substituted complexes. The MO calculations on Re(CO)4[Re(CO)4(bma)] (3) reveal that the HOMO and LUMO levels are formed from the HOMO of the d8-ML4 fragment Re(CO)4− and its interaction with the ψ4π* antibonding MO of the bma ligand on the [Re(CO)4(bma)]+ fragment.