First stable reduced form of [Co(5)](+10): fine tuning of linear pentacobalt(ii) complexes containing delocalized metal-metal bonds through ligand modification.

Pentacobalt EMACs [Co(5)(mu(5)-dpzpda)(4)X(2)] (X = Cl(-) (), NCS(-) ()) with fine-tuning of the supporting ligand based on the tripyridyldiamine ligand, N,N'-di(pyrazin-2-yl)pyridine-2,6-diamine (H(2)dpzpda), and their reduced form (Ph(4)P)[Co(5)(mu(5)-dpzpda)(4)X(2)] (X = Cl(-) (), NCS(-) ()) were first synthesized and structurally characterized. The structures of showed direct Co-Co bonds with Co-Co distances in the range 2.2385(7)-2.2888(15) A, and valence electrons delocalized through the whole metal chain with distances of longer than 9.06 A. The distances of the inner Co-Co bonds and the Co-axial ligands became longer after reduction, whereas no significant change was observed in the distances of the outer Co-Co bonds and Co-N (supporting ligand) bonds, which was consistent with the MO analysis. Electrochemical studies on both and showed one reversible oxidation and one reversible reduction at E(1/2) = +0.82 and -0.05 V for , and at E(1/2) = +0.89 and +0.02 V for , respectively. The redox reactions of the thiocyanate complex happened at higher potentials than the chloride complex . A magnetism study of revealed anomalous magnetic behaviour similar to that of heptacobalt EMACs, and a deviation from the Curie-Weiss law was observed. The chi(M)T value at 300 K is 0.84 and 1.16 emu K mol(-1) for and , respectively, suggesting spin-equilibrium or a spin-admixture between doublet and quartet states arising from the Boltzmann distribution over different energy levels. Similar results were obtained for and , showing intermediate chi(M)T values between a diamagnetic and a triplet state of 0.15-0.96 emu K mol(-1) in the temperature range 5-300 K. The structural and magnetic results were interpreted through an EHMO study.