Mixed-valence [Fe(I)Fe(II)] hydrogenase active site model complexes stabilized by a bidentate carborane bis-phosphine ligand. 
A series of [FeFe]-hydrogenase active site analogues, with the general formula [Fe(2)(dt)(CO)(4)(BC)] 1-3 (dt = dithiolate, pdt = propyl-1,3-dt (1), bdt = benzene-1,2-dt (2), edt = ethyl-1,2-dt (3); BC = 1,2-bisdiphenylphosphine-1,2-o-carborane), has been prepared and structurally characterized.  While the electrochemical reductions of 1-3 are largely invariant to the different nature of their dt bridges, the oxidations differ by more than 120 mV in between the series.  Remarkably, all three compounds are reversibly oxidized, with complex 1 that contains the most electron-donating pdt ligand at the mildest potential of -0.09 V vs. Fc/Fc(+).  The one-electron oxidized state 1(ox) is stable for several minutes and was spectroscopically characterized by FTIR and EPR.  EPR spectroscopy provided evidence that in the mixed-valence [Fe(I)Fe(II)] state most of the spin density is located on the iron with the BC-ligand.  This is monitored through the strong (31)P hyperfine coupling of the phenyl groups of the BC ligand, while further delocalization into the o-carborane unit is negligible.