Dirhodium complexes with biimidazole (H(2)bim) ligands [Rh-2(O2CR)(2)(H(2)bim)(2)Cl-2] (R = Bu ([1Cl(2)]), Pr ([2Cl(2)])), [Rh-2(O2CBu)(2)(H(2)bim)(2)](PF6)(2) ([1](PF6)(2)), [Rh-2(O2CBu)(2)(H(2)bim)(2)(PPh3)(2)](PF6)(2) ([1(PPh3)(2)](PF6)(2)), and [Rh-2(O2CPr)(2)(H(2)bim)(2)(PPh3)(2)]Cl-2 ([2(PPh3)(2)]Cl-2) have been synthesized. Deprotonation of the biimidazole complexes afforded the quadruply hydrogen-bonded dimers of the biimidazolate complexes [Rh-2(O2CR)(2)(Hbim)(2)(PPh3)(2)](2) (R = Bu ([1'(PPh3)(2)](2)) and Pr ([2'(PPh3)(2)](2))). Complementary hydrogen bonds between the Hbim ligands are not coplanar because the Hbim ions in one dirhodium complex are not parallel (the dihedral angle between them is ca. 15 degrees). A cyclic voltammogram of [1'(PPh3)(2)](2) shows two sets of two consecutive oxidation waves in CH2Cl2. The one-electron-oxidized species of [1'(PPh3)(2)](2) showed no intervalence charge-transfer band in the electronic spectrum and an axially symmetrical ESR spectrum with hyperfine structure because of two phosphorus atoms. These observations show that the odd electron is localized in a s(RhRh) orbital on one dirhodium unit. Theoretical calculations indicate that an oxidized complex [Rh-2(O2CMe)(2)(bim)(2)(PMe3)(2)](-) hydrogen bonded with a biimidazole complex [Rh-2(O2CMe)(2)(H(2)bim)(2)(PMe3)(2)](2+) was a stable mixed-valence complex.