The Co(OESPz) complex crystallizes in space group P2(1)/n, with a = 10.260(5) Angstrom, b = 22.650(5) Angstrom, c = 16.720(5) Angstrom, beta = 91.680(5)degrees, and Z = 4 and forms, similarly to the isomorphous Mn(OESPz), solid-state extended one-dimensional aggregates where "ruffled" molecular units are held together by extra planar Co ... S interactions. The complex, which contains a low-spin Co2+ (S = 1/2) ion, exhibits intermolecular ferromagnetic interactions (Theta = 2.6 K, g = 2.44) propagated through a superexchange pathway by extra planar Co S interactions, with a possible contribution of the pi system. Co(OESPz) shows a Co-N-p distance shorter than CoOEP and CoPc and a sensible electrochemical stabilization of the Co2+ vs Co3+ state. The bonding interactions between Co2+ and the macrocyclic ligands OMSPz(2-), Pc2-, Pz(2-), and P2- are analyzed in detail, within the density functional (DF) theory, to elucidate the effects of the ligand framework. It is concluded that the cr interactions, which are by far dominant in all members of the Co-II tetrapyrrole series, in the aza-bridged complexes, particularly in CoPz and Co(OMSPz), are stronger than in CoP, due to the reduced "hole" size of the macrocycle. The JE interactions, consisting of pi back-donation from Co-3d(pi) into empty ring pi orbitals and donation from the occupied ring pi orbitals into the Co-4p(z) are rather weak, but there is a sizable contribution from polarization of the macrocyclic ligand. The aza bridges have little effect on the metal to ligand pi back-donation which in Co(OMSPz) is completely absent; the peripheral substituents, which are responsible for large polarization effects, play a more relevant role in the metal-macrocycle pi interactions. In all porphyrazines the total orbital interaction contribution (covalent component) prevails over the ionic component of the bond, the latter being identified as the sum of the Pauli repulsion and the attractive electrostatic interaction between Co2+ and the tetrapyrrole(2-).