Metal ion function depends on the regulation of properties within the primary and second coordination spheres. An approach toward studying the structure-function relationships within the secondary coordination sphere is to construct a series of synthetic complexes having constant primary spheres but structurally tunable secondary spheres. This was accomplished through the development of hybrid urea-carboxamide ligands that provide varying intramolecular hydrogen bond (H-bond) networks proximal to a metal center. Convergent syntheses prepared ligands [(N'-tert-butylureayl)-N-ethyl]-bis(N"-R-carbamoylmethyl) amine (H(4)1(R)) and bis[(N'-tert-butylureayl)-N-ethyl]-(N"-R-carbamoylmethyl) amine (H(5)2(R)), where R = isopropyl, cyclopentyl, and (S)-(-)-alpha-methylbenzyl. The ligands with isopropyl groups H(4)1(iPr) and H(5)2(iPr) were combined with tris[(N'-tert-butylureayl)-N-ethyl] amine (H(6)buea) and bis(N-isopropylcarbamoylmethyl)amine (H(3)0(iPr)) to prepare a series of Co(II) complexes with varying H-bond donors. [Co(II)H(2)2(iPr)]-(two H-bond donors), [Co(II)H1(iPr)]-(one H-bond donor), and [(CoOiPr)-O-II](-) (no H-bond donors) have trigonal monopyramidal primary coordination spheres as determined by X-ray diffraction methods. In addition, these complexes have nearly identical optical and EPR properties that are consistent with S = (3)/(2) ground states. Electrochemical studies show a linear spread of 0.23 V in anodic potentials (E-pa) with [Co(II)H(22)iPr]-being the most negative at -0.385 V vs [Cp2Fe](+)/[Cp2Fe]. The properties of [Co(II)H(3)buea](-) (H(3)buea, tris[(N'-tertbutylureaylato)N-ethyl] aminato that has three H-bond donors) appears to be similar to that of the other complexes based on spectroscopic data. [CoIIH3buea](-) and [(CoH22iPr)-H-II]-react with 0.5 equiv of dioxygen to afford [Co(III)H(3)buea(OH)](-) and [(CoH22iPr)-H-III(OH)](-). Isotopic labeling studies confirm that dioxygen is the source of the oxygen atom in the hydroxo ligands: [Co(III)H(3)buea((OH)-O-16)](-) has a v(O-H) band at 3589 cm(-1) that shifts to 3579 cm(-1) in [Co(III)H(3)buea((OH)-O-18)](-); [(CoH22iPr)-H-III(OH)](-) has v(O-16-H)) 3661 and v(O-18-H)) 3650 cm(-1). [Co(II)H1(iPr)](-) does not react with 0.5 equiv of O-2; however, treating [Co(II)H1(iPr)](-) with excess dioxygen initially produces a species with an X-band EPR signal at g = 2.0 that is assigned to a Co-O-2 adduct, which is not stable and converts to a species having properties similar to those of the Co-III-OH complexes. Isolation of this hydroxo complex in pure form was complicated by its instability in solution (k(int)) 2.5 x 10(-7) M min(-1)). Moreover, the stability of the Co-III-OH complexes is correlated with the number of H- bond donors within the secondary coordination sphere; [Co(III)H(3)buea(OH)](-) is stable in solution for days, whereas [(CoH22iPr)-H-III(OH)](-) decays with a k(int) = 5.9 x 10(-8) M min(-1). The system without any intramolecular H- bond donors [(CoOiPr)-O-II](-) does not react with dioxygen, even when O2 is in excess. These findings indicate a correlation between dioxygen binding/activation and the number of H- bond donors within the secondary coordination sphere of the cobalt complexes. Moreover, the properties of the secondary coordination sphere affect the stability of the Co-III-OH complexes with [Co(III)H(3)buea(OH)](-) being the most stable. We suggest that the greater number of intramolecular H- bonds involving the hydroxo ligand reduces the nucleophilicity of the Co-III-OH unit and reinforces the cavity structure, producing a more constrained microenvironment around the cobalt ion.