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A new mixed-valence one-dimensional coordination polymer of formula {[Co^II(MeOH)₂][(<i>μ</i>-NC)₂Co^III(dmphen)<i>(</i>CN)₂]₂}_n·2nH₂O (<b>1</b>) was obtained by reacting the Ph₄P[Co^II(dmphen)(CN)₃] metalloligand (dmphen = 2,9-dimethyl-1,10-phenanthroline and Ph₄P⁺ = tetraphenylphosphonium ion) with cobalt(II) acetate tetrahydrate. The structural analysis shows the formation of a neutral 4,2-ribbon-like chain of vertex-sharing cyanido-bridged {Co^III₂Co^II₂} squares in which the metalloligand underwent an oxidation process and a reorganization to form {Co^III(dmphen)(CN)₄}⁻ linkers that coordinate to the [Co^II(MeOH)₂]²⁺ units through single cyanido ligands. Both cobalt(II) and Co(III) cations are six-coordinated in distorted octahedral environments. The shortest intrachain distance between the paramagnetic cobalt(II) ions is 7.36 Å, a value which is shorter than the shortest interchain one (10.36 Å). Variable-temperature (1.9–300 K) static (dc) magnetic measurements for <b>1</b> indicate the occurrence of magnetically isolated high-spin cobalt(II) ions with a D value of +67.0 cm⁻¹. Dynamic alternating current (ac) magnetic measurements between 2.0–13 K reveal that <b>1</b> exhibits slow magnetic relaxation under non-zero applied dc fields, being thus a new example of field-induced SIM with easy-plane magnetic anisotropy. Theoretical calculations by CASSCF/NEVPT2 on <b>1</b> support the results from magnetometry. The relaxation of the magnetization occurs in the ground state under external dc fields through a two-phonon Raman process and one intra-Kramers mechanism.