The reaction of 2,2'-bipyridine-3,3'-diol (H2L) and cis-Os-II(bpy)(2)Cl-2 (bpy = 2,2'-bipyridine) results in isomeric forms of [Os-II(bpy)(2)(HL-)]ClO4, [1]ClO4 and [2]ClO4, because of the varying binding modes of partially deprotonated HL-. The identities of isomeric [1]ClO4 and [2]ClO4 have been authenticated by their single crystal X-ray structures. The ambidentate HL- in [2]ClO4 develops the usual N,N bonded five-membered chelate with a strong O-H center dot center dot center dot O hydrogen bonded situation (O-H center dot center dot center dot O angle: 160.78 degrees) at its back face. The isomer [1]ClO4 however represents the monoanionic O-,N coordinating mode of HL-, leading to a six-membered chelate with the moderately strong O-H center dot center dot center dot N hydrogen bonding interaction (O-H center dot center dot center dot N angle: 148.87 degrees) at its backbone. The isomeric [1]ClO4 and [2]ClO4 also exhibit distinctive spectral, electrochemical, electronic structural, and hydrogen bonding features. The pK(a) values for [1]ClO4 and [2]ClO4 have been estimated to be 0.73 and <0.2, respectively, thereby revealing the varying hydrogen bonding interaction profiles of O-H center dot center dot center dot N and O-H center dot center dot center dot O involving the coordinated HL-. The O-H center dot center dot center dot O group of HL- in 2(+) remains invariant in the basic region (pH 7-12), while deprotonation of O-H center dot center dot center dot N group of HL- in 1(+) estimates the pK(b) value of 11.55. This indeed has facilitated the activation of the exposed O-H center dot center dot center dot N function in [1]ClO4 by the second {Os-II(bpy)(2)} unit to yield the L2- bridged [(bpy)(2)Os-II(mu-L2-)Os-II(bpy)(2)](ClO4)(2) ([3](ClO4)(2)). However, the O-H center dot center dot center dot O function in [2]ClO4 fails to react with {Os-II(bpy)(2)}. The crystal structure of [3](ClO4)(2) establishes the symmetric N,O-/O-,N bridging mode of L2-. On the other hand, the doubly deprotonated L'(2-) (H2L' = 2,2'-biphenol) generates structurally characterized twisted seven-membered O-,O- bonded chelate (torsion angle >50 degrees) in paramagnetic [Os-III(bpy)(2)(L'(2-))]ClO4 ([4]ClO4). The electronic structural aspects of the complexes reveal the noninnocent potential of the coordinated HL-, L2-, and L'(2-). The K-c value of 49 for 3(3+) reveals a class I mixed-valent (OsOsIII)-Os-II state.