STUDIES ON THE INTERACTION BETWEEN LANTHANIDE IONS AND LIGANDS IN SOLID COMPLEXES OF LANTHANIDE HEXACYANOMETALATES - PART III - SYNTHESIS AND THERMAL-PROPERTIES OF A SERIES OF LANTHANIDE HEXACYANOCOBALTATE(III) N-METHANOLS

A series of lanthanide hexacyanocobaltate(III) n-methanols, Ln[Co(CN)( 6)].nMeOH, where MeOH means methanol, were prepared in order to gain s ome insight into the relation between crystal structure and the therma l desolvation of methanol of these complexes. The desolvation of metha nol of the complex was followed by means of TG-DTA, DSC, X-ray powder diffraction analysis and Raman spectra. It was confirmed from thermogr avimetry 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 c onditions. Therefore, the complex, Ce[Co(CN)(6)].2.5MeOH was the bound ary complex having MeOH molecules between 2.5 and 2, and the result co ntrasted with those of the hydrate series complexes, i.e. Nd[Co(CN)(6) ].5H(2)O is the boundary between penta hydrates and tetra hydrates. Th e TG curves show that the desolvation of Ln[Co(CN)(6)].2.5MeOH takes p lace in at least three stages as follows. Ln[Co(CN)(6)].2.5MeOH-->Ln[C o(CN)(6)].2MeOH + 0.5MeOH (1) Ln[Co(CN)(6)].2MeOH-->Ln[Co(CN)(6)].0.5M eOH + 1.5MeOH (2) Ln[Co(CN)(6)].0.5MeOH-->Ln[Co(CN)(6)] + 0.5MeOH (3) The desolvation of the complexes from Pr to Lu[Co(CN)(6)].2MeOH, consi sts of the stages Eqs. (2) and (3). For the stage (Eq. (2)) of the ser ies 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 o f the complex was explained by the combination of two contrary factors , i.e. (i) decrease of the ionic radius of Ln(3+) enhances the interac tion of Ln(3+)-MeOH bonds, and (ii) decrease of the ionic radius of Ln (3+) leads to a narrowing of the coordination sphere and then a mutual repulsion between the ligands contributes to the instability of Ln(3)-MeOH bonds. (C) 1998 Elsevier Science S.A.