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Unusual Thermodynamic Behaviour on Complexation of Cobalt(II) with Chloride, Bromide and Iodide Ions in Hexamethylphosphoric Triamide

DOI: 10.1039/f19898503747

Source and publish data:

Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases p. 3747 - 3756 (1989)

Update date:2022-08-04

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Authors:

Abe, YurikoAbe, Yuriko

Ozutsumi, KazuhikoOzutsumi, Kazuhiko

Ishiguro, Shin-IchiIshiguro, Shin-Ichi

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Article abstract of DOI:10.1039/f19898503747

The complexation of cobalt(II) with chloride, bromide and iodide ions has been studied by spectrophotometry and calorimetry in hexamethylphosphoric triamide (HMPA) containing 0.1 mol dm-3 (n-C4H9)4NClO4 as a constant ionic medium at 25 deg C.The formation of n>(2-n)+ (n = 1-4 with X as Cl, 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 n(HMPA)4-n>(2-n)+ (n = 1-4) are formed stepwise.In spite of the remarkably strong donicity of HMPA compared to other aprotic donor solvents, the ΔH10 values of formation of + are even smaller, or more negative, in HMPA than in the others.Indeed, for + unusual exothermicity 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 2+ is considerably crowded for bulky HMPA molecules, and Co-O(HMPA) bonds are thus elongated to some extent.

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