Infra-red and visible absorption studies of the pentacyanonickelate(II) ion in aqueous solution

Abstract

The infra-red spectra of aqueous solutions containing tetracyanonickelate(II) and excess cyanide ions show a single new absorption peak characteristic of the pentacyanonickelate(II) ion, [Ni(CN) 5] 3−. The growth of this new infra-red peak and the diminution of the known [Ni(CN) 4] 2− absorption is described completely by the equilibrium: [Ni(CN) 4] 2− + CN − ⇋ [Ni(CN) 5] 3− over a cyanide concentration range 0·05-5 M. No infra-red absorption of ahigher complex, e.g. [Ni(CN) 5] 4− was detected even in nearly saturated NaCN and ∼0·4 M [Ni(CN) 4] 2−. Infra-red absorption characteristics of the complex ions are: [ Ni(CN) 4] 2−, ∈ = 1068 ± 95 mole −1 l. cm−1 at 2124 ± 1 [ Ni(CN) 5] 3−, ∈ = 1730 ± 230 mole −1 l. cm−1 at 2102 ± 2 Continuous variation experiments at 100 Å intervals from 3800 Å to 6000 Å showed only a 1 : 1 complex between CN − and [Ni(CN) 4] 2−. From visible and infra-red absorption data the constant for the formation of [Ni(CN) 5] 3− from [Ni(CN) 4] 2− and CN − was found to be 0·19 ± 0·01 1. mole −1 at 25·2° in solutions of constant ionic strength (μ = 1·34). The formation constant was determined at three temperatures over the range 15–35°, yielding a ΔH of ∼ −3 kcal/mole. Magnetic measurements show that the pentacyanonickelate(II) ion is diamagnetic.