5-[4-(N-tert-butyl-N-aminoxyl)phenyl]pyridimine (4NITPhPyrim = RL) forms a 1-D ladder polymer complex with Co(hfaC)(2) of stoichiometry Co-3(RL)(2)(hfaC)(6), having antiparallel [(CoRL)-R-II](n) linear chains (rails) that are cross-linked by Pyrim-Co(hfaC)(2)-Pyrim rungs. The magnetic behavior above 100 K is consistent with contributions from one high-spin Co-II ion (the cross-link, S = (3)/(2)) plus two Co-ON units with strongly antiferromagnetic (AFM) metal-radical exchange (each S = 1). The chi T data show an AFM downturn as the temperature drops. Assuming weak exchange along chain portions of the polymer due to poor spin polarization across the phenyl-pyrimidine bond in RL, a linear three-spin (S = 1, (3/)(2), and 1) fit to the T > 18 K data yields an AFM cross-linker (rung) effective exchange of J(CL)/k = (-)5.3 K = (-)3.7 cm(-1). Superexchange (sigma-orbital overlap) is a likely mechanism for the effective AFM exchange between CoON and Co spin sites in the three-spin groupings.