Compounds of Dipositive Cobalt

Divalent cobalt may assume a coordination number of either 4 or 6 in its complexes. The large majority of the tetracovalent complexes, including such simple ones as C0F72, CoBr72, and Co(SCX)72, are tetrahedral, but a few of the more complicated Co(II) chelates are known to be planar. The majority of the hexacovalent complexes, such as Co(NH )t2, Co(H20)f, and Co(C204)74, are of the spin-free, rather than the spin-paired, type. As indicated below, a spin-paired octahedral complex of Co(II) has one electron beyond a sextet of low-energy 3d orbitals  

Recently it has been found that the cyano complex of Co(II) may be converted (by the action of such powerful reducing agents as potassium amalgam or NaBH4) to the ion HCo(CN)73. The cobalt in this complex 

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