A series of lanthanide
hexacyanocobaltate(III) n-methanols, Ln[Co(CN)
6]·nMeOH, where MeOH means methanol, were prepared in order to gain some insight into the relation between crystal structure and the thermal desolvation of methanol of these
complexes. The desolvation of methanol of the
complex was followed by means of TG–DTA, DSC, X-ray powder diffraction analysis and Raman spectra. It was confirmed from thermogravimetry that the
complexes with La and Ce form Ln[Co(CN)
6]·2.5MeOH and those with Pr to Gd form Ln[Co(CN)
6]·2MeOH. The Ce[Co(CN)
6]·2.5MeOH and Ce[Co(CN)
6]·2MeOH were obtained by different preparation conditions. Therefore, the
complex, Ce[Co(CN)
6]·2.5MeOH was the boundary
complex having MeOH molecules between 2.5 and 2, and the result contrasted with those of the hydrate series
complexes, i.e. Nd[Co(CN)
6]·5H
2O is the boundary between penta hydrates and tetra hydrates. The TG curves show that the desolvation of Ln[Co(CN)
6]·2.5MeOH takes place in at least three stages as follows.
Ln[Co(CN)6]·2.5MeOH→Ln[Co(CN)6]·2MeOH+0.5MeOHLn[Co(CN)6]·2MeOH→Ln[Co(CN)6]·0.5MeOH+1.5MeOHLn[Co(CN)6]·0.5MeOH→Ln[Co(CN)6]+0.5MeOHThe desolvation of the complexes from Pr to Lu[Co(CN)6]·2MeOH, consists of the stages Eq. 2 and Eq. 3.
For the stage (Eq. 2) of the series complexes, the temperatures rise going from La to Pr complexes and gradually fall from Pr to Lu. The trend of the enthalpy change of the overall process for the series of the complexes is sharper than that of the temperature change. This result shows that the structure of the methanol-solvated complex with Pr is the most stable. The stability of the complex was explained by the combination of two contrary factors, i.e. (i) decrease of the ionic radius of Ln3+ enhances the interaction of Ln3+–MeOH bonds, and (ii) decrease of the ionic radius of Ln3+ leads to a narrowing of the coordination sphere and then a mutual repulsion between the ligands contributes to the instability of Ln3+–MeOH bonds.
