Tartrate bridging ligands in divalent Co(II)/Co(III) dinuclear anion

A cobalt(II/III) ionic compound with bridging tartaric acid (H4L) anion (CH6N3)3[Co2II/III(L)2(H2O)2]center dot 4H2O 1 was synthesized. The crystallites were isolated from highly alkaline aqueous solution in the presence of guanidinium cations (CH6N3)+ being then incorporated as counter ions. A determined structure reveals the dinuclear coordination anion with two cobalt centers, each within its distinctive coordination octahedron CoO6 sphere showing different Co-O bond lengths. The metal center with shorter coordination bonds is assigned as Co(III), while the other with Co(II), all in agreement with overall net charge equilibrium. This involves three guanidinium cations, two tartrate(4-) anions, the latter within the coordination anion, as well as with bond valence sums for both cobalt centers. Both tartrates (L) are triply bonded on the Co(III) center, thus enabling CoIIIO6 coordination bonds. On the other hand, only two coordination bonds are enabled by each tartrate towards Co(II), while the remaining two in CoIIO6 are completed by two terminal water molecules. The electronic spectrum shows clearly the d-d bands at 400 and 665 nm for Co(III), while at 520 nm for Co(II), for each CoO6, respectively. The temperature dependent magnetic susceptibility shows the paramagnetic behavior of the compound, due to the d7 Co(II) ions being in a high spin state, while the d6 low spin Co(III) ions are diamagnetic.