A series of tetradentate 2,2':6',2":6",2"-quaterpyridine-type ligands related to ppq (ppq = 8-(1",10"phenanthrol-2"-y1)-2-(pyrid-2'-yOquinoline) have been synthesized. One ligand replaces the 1,10-phenanthroline (phen) moiety of ppq with 2,2'-bipyridine and the other two ligands have a 3,3'-polymethylene subunit bridging the quinoline and pyridine. The structural result is that both the planarity and flexibility of the ligand are modified. Co (II) complexes are prepared and characterized by ultraviolet-visible light (UVvis) and mass spectroscopy, cyclic voltammetry, and X-ray analysis. The light-driven H-2-evolving activity of these Co complexes was evaluated under homogeneous aqueous conditions using [Ru(bpy)(3)](2) as the photosensitizer, ascorbic acid as a sacrificial electron donor, and a blue light-emitting diode (LED) as the light source. At pH 4.5, all three complexes plus [Co(ppq)Cl-2] showed the fastest rate, with the dimethylene-bridged system giving the highest turnover frequency (2125 h(-1)). Cyclic voltammograms showed a significant catalytic current for H2 production in both aqueous buffer and H2O/DMF medium. Combined experimental and theoretical study suggest a formal Co(L)-hydride species as a key intermediate that triggers H-2 generation. Spin density analysis shows involvement of the tetradentate ligand in the redox sequence from the initial Co(II) state to the Co(II)-hydride intermediate. How the ligand scaffold influences the catalytic activity and stability of catalysts is discussed, in terms of the rigidity and differences in conjugation for this series of ligands.