The effects of the substituents on the chelating ligands located in the secondary coordination sphere on the 0, affinity of cobalt(II) centers have been explored. The combination of facially capping tridentate tris(pyrazolyl)borates (= Tp(Me2,4R)) and bidentate bis(imidazolyl)borates (= [B(Im(N-Me))(2)MeX](-) ; L-x) yields square pyramidal cobalt(II) complexes. The structural properties of the substituent groups X attached to the boron center of L-x affect the arrangement of X in the resulting cobalt(II) complexes [Co-II(Tp(Me2,4R))- (L-x)]. When the boron-attached moiety of X is a relatively bulky sp(3-)CH(2)Y group (i.e., X:Y = Me:H and nBu:nPr), the alkyl group X faces the cobalt center, whereas for isopropoxy (OiPr) and phenyl (Ph) groups, of which the boron-attached atoms are a less hindered oxygen atom and a planer sp(2)-carbon, respectively, the X group is arranged away from the cobalt center. This flexible behavior of Lx is reflected in the O-2 affinity of the cobalt(II) center, which depends on the extent to which the complex sphere is shielded by the ligands. The dependence of the cobalt(II) oxidation potential on the X substituent of L-x is inconsistent with the O-2 affinity. On the other hand, the electronic properties of R, which is attached to the fourth position of the pyrazolyl rings in the rigid Tp(Me2,4R) ligand, are reflected in the electrochemical properties and O-2 affinity of the cobalt center.