Potentiometric and spectrometric investigations of cobalt (II) - Ethanolamine (mono, di and tri) complexes

Abstract

A detailed potentiometric and spectrophotometric investigation of cobalt-ethanolamine (mono-, di- and tri-) complexes has been made. Potentiometric investigations have been carried out at various metal ion concentrations and also at various concentrations of ethanolammonium ion. In cobalt(II)-monoethanolamine system, the formation curves (i.e., n–pA curves) taken at different metal concentrations and in presence of 0.1 to 1.0 M monoethanolammonium ion solutions coincide, indicating the formation of only mononuclear complexes. Evidence is obtained for the formation of the following pure and mixed hydroxy complexes: CoM²⁺ CoM⁺ CoM₂⁺ CoM₃⁺ The stability constants of these complexes have been determined by the linear plot of the formation function for pure and mixed hydroxy complexes. Similar experiments have been carried out in the diethanolamine and triethanolamine systems. The formation of the following complexes has been established and the stability constants have been determined by the analysis of n–pA curves: CoD²⁺ CoD₂⁺ CoT²⁺ CoT(OH)⁺ CoT(OH)₂ It is noticed in the case of the triethanolamine system that n–pA curves at various metal concentrations do not coincide in presence of 0.01 M triethanolammonium ion, indicating the formation of polynuclear complexes. On the basis of ‘core + links’ theory of Sillen, Co(TOHCo)³⁺ or Co₂TOH³⁺ has been suggested as a plausible formula. Absorption spectra of the aquo cobalt ion and cobalt-ethanolamine (mono-, di- and tri-) system at various n values have been taken in presence of 1 M ethanolammonium (mono-, di- and tri-) nitrate. The spectra of pure complexes is obtained by calculating the percentage of the different complex species from the stability constants of the complexes from potentiometric data and using the equation: εₘ = Σ(αᵢεᵢ) = α₀ε₀ + α₁ε₁ + α₂ε₂ + … It has been possible to obtain the spectra of pure Co²⁺, CoM₂²⁺, CoM₃²⁺, CoD²⁺ and CoT²⁺. In the case of aquo cobalt ion in 1 M ethanolammonium (mono-, di- or tri-) nitrate, the main visible band is located at ca. 515 nm. In addition, a weak band occurs at ca. 625 nm. In the diethanolamine system there is also a weak band at about 900 nm. To fix the two absorption bands (at 515 nm and 625 nm), Dq and ΔE(⁴T₁g–⁴F) are used as parameters and their values calculated from the determinantal equation of the energy matrix for the two ⁴T₁g levels. The main band at ca. 515 nm corresponds to ⁴T₁g(F) → ⁴T₁g(P) and the band at ca. 625 nm corresponds to ⁴T₁g(F) → ⁴A₂g(F). The band at ca. 625 nm is extremely weak since it represents a two-electron jump. The band corresponding to ⁴T₁g(F) → ⁴T₂g(F) lying beyond 1000 nm has also been calculated. It is suggested that the band at about 900 nm may be due to the spin-forbidden transition ⁴T₁g(F) → ²E_g. The main band has a shoulder on the low-energy side which may be due to intermixing of ⁴T₁g(F) → ⁴T₁g(P) with the spin-forbidden transition states derived from ²B and ²G. The behaviour of the ethanolamine (mono-, di- and tri-) complexes is very interesting. The broad features are practically the same as those of aquo complexes. Due to decrease in symmetry (tetragonal in the case of 1:1 and 1:2 complexes and rhombic in the case of trans form of CoM₃²⁺), degeneracy of the various states is lifted and the spectra tends to become more complicated. In the case of CoD²⁺ and CoT²⁺, the main visible band comprises of two peaks in addition to the shoulder. The shoulder could possibly be due to the mixing up of the spin-forbidden transition to the doublet states with the main transition. Two peaks appear due to the splitting up of ⁴T₁g(P) state into two in the tetragonal field. The band appearing at about 625 nm evidently corresponds to ⁴T₁g(F) → ⁴A₂g(F) which appears in the aquo ion. The weak band appearing at about 900 nm can be considered as corresponding to the spin-forbidden transition to ²E_g noticed in aquo cobaltous ion. It is also possible to treat this band along with the tail of another band as due to tetragonal effect. In CoT²⁺ the appearance of two peaks on the main band is not markedly observed although the shoulder is seen. Charge-transfer bands are noticed in cobalt-monoethanolamine and cobalt-diethanolamine systems. The increase in intensity of the main visible band in CoM₃²⁺ appears to be due to intensity stealing from the neighbouring charge-transfer band. The general features of the absorption bands obtained at various n values have also been explained.