Unusual thermodynamic behaviour on complexation of cobalt(II) with chloride, bromide and iodide ions in hexamethylphosphoric triamide

Abstract

The complexation of cobalt(II) with chloride, bromide and iodide ions has been studied by spectrophotometry and calorimetry in hexamethyl-phosphoric triamide (HMPA) containing 0.1 mol dm^–3(n-C₄H₉)₄NCIO₄ as a constant ionic medium at 25 °C. The formation of [CoX_n]^(2–n)(n= 1–4 with X as CI, n= 1–3 with X as Br and n= 1 and 2 with X as I) is proposed, and their formation constants, enthalpies and entropies are determined. Electronic spectra of individual cobalt(II) halogeno complexes are also extracted, indicating that the cobalt(II) ion is four-coordinated in HMPA(unlike other oxygen-donor solvents which show six-coordination), and four-coordinate tetrahedral [CoX_n(HMPA)_4–n]^(2–n)+(n= 1–4) are formed stepwise. In spite of the remarkable strong donicity of HMPA compared to other aprotic donor solvents, the ΔH^°₁values of formation of [CoX]₊ are even smaller, or more negative, in HMPA is observed in contrast to endothermicity in N,N- dimethylformamide or dimethyl sulphoxide. This suggests that, although being sterically allowed, the four-coordination of [co(HMPA)₄]²⁺is considerable crowded for bulky HMPA molecules, and Co—O(HMPA) bonds are thus elongated to some extent.