The coordination structure of the cobalt(II) ion and its chloro complexes of low concentration (∼20 mmol dm-3) in hexamethylphosphoric triamide (HMPA) has been explored by laboratory EXAFS (extended X-ray absorption fine structure) modified for fluorescence detection. The cobalt(II) ion is four-coordinated in HMPA, unlike other oxygen-donor solvents such as water and N,N-dimethylformamide (DMF). The CoO (HMPA) bond lengths within complexes are revealed to be 194(1), 195(1), 194(1) and 202(2) pm for [Co(hmpa)4]2+, [CoCl(hmpa)3]+, [CoCl2(hmpa)2] and [CoCl3(hmpa)]-, respectively. The CoO bond length is significantly shorter than that within six-coordinate [Co(H2O)6]2+ (208 pm) or [Co(dmf)6]2+ (206 pm), and remains practically unchanged upon formation of mono- and dichloro complexes. On the other hand, the CoCl bond length is 224(1), 227(1) and 229(1) pm within [CoCl(hmpa)3]+, [CoCl2(hmpa)2] and [CoCl3(hmpa)]-, respectively, all of which are appreciably shorter than that within the six-coordinate [CoCl(H2O)5]+ (235 pm). The origin of the different thermodynamic behavior of chloro complexation of cobalt(II) in HMPA from that in DMF will be discussed on the basis of these structural data.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry