Low-spin cobalt(II) complexes. Part II. Cyanide complexes

Abstract

The complexes present in freshly-prepared solutions containing cobalt(II) and cyanide ions have been investigated. The main complex is identified as the hexaco-ordinate [Co(CN)₅H₂O]^3– by comparison of its spectrum with those of the analogous isocyanide complexes. No evidence was found for the hexacyanide [Co(CN)₆]^4– under equilibrium conditions (K₆ 1 at µ= 2), but another complex, probably the tetracyanide [Co(CN)₄(H₂O)₂]^2–, is formed in very dilute solutions. The alkali-metal cations form ion-pairs with the pentacyanide; the ion-pair with Rb⁺ is identified as [Rb,Co(CN)₅H₂O]^2– and its formation constant in 3N-(Rb,Na)Cl at 20° is 3·7 l./mole. In 3N-LiCl a dimeric complex is also formed. Factors determining the stoicheiometry and stereochemistry of the cobalt(II) complexes with cyanide and isocyanide ligands are discussed.