The Relative Stability of Octahedral and Tetrahedral Complexes of Transition Metal Ions
It is well known that the relative stability of octahedral and tetrahedral complexes in solution depends on the properties of ligands in addition to being dependent on the central metal ion. for a discussion of the latter effect it is desirable to possess
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THE RELATIVE ST ABILITY OF OCT AHEDRAL AND TETRAHEDRAL COMPLEXES OF TRANSITION METAL IONS. Irmina Uruska and Wl'odzimierz Libus Department 6f Physical Chemistry, Technical University of Gdafisk, Gdafisk, Poland.
It is weH known that the relative stability of octahedral and tetrahedral complexes in solution depends on the properties of ligands in addition to being dependem on the central metal ion. For a discussion of the latter effect it is desirable to possess data concerning the complexes of different metals with the same ligands. With the object of obtaining equilibrium constants fulfilling this requirement an investigation of equilibria of the type: V. Gutmann (ed.), Proceedings of the 8th International Conference on Coordination Chemistry © Springer-Verlag Wien 1964
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{1} M - being a divalent transition metal ion - has been undertaken in the present work. In the case of cobalt(ll) this equilibrium was investigated by KATZIN (1) and quite recently by KING et ale (2). This werk is a continuation of earlier investigations performed by one of us (3). A method of determination of the equilibrium constant of {I} based on measurements of solubilities has been divesed. So far equilibria involving Co(II}. Ni{II). and Zn(U} were investigated. chlorobenzene being used as an inert solvent. The determined solubility isotherms for chlorobenzene-pyridine mixtures are shown in Fig. 1. the compositions of solid phases in equilibrium being indicated. It i5 seen that solid complexes are very slightly soluble in pure chlorobenzene but their solubility increases more or less rapidlyon increasing pyridine concentration. This effect is a result of the formation of
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- 342 octahedral complexes MCl2PY4 in solution according to {l}, in addition to a presumably smaller and nonspecific effect of solvent composition o~ the activity coefficients Y0 of individual complexes. For an evaluation of equilibrium constants from solubility data the second effect must first be considered. For the octahedral complex NiCl2PY4 the dependence of Yo on solvent composition may be found directly from solubility data, owing to the fact that, except for smallest pyridine concentrations, practically a11 nickel(II} in solution is in the form of NiCI2PY4. A similar dependence on Yn on solvent composition for CoC~PY4 may be calculated from both solubility and optical density measurements the latter enabling for a direct determination of concentrations of CoC~PY2 and CoCl2PY4 complexes in solution. As a final result of these calculations it has been found that Yn is the same function of solvent composition (chlorobenzene + pyridine mixtures) for both NiC~PY4 and CoC~PY4 octahedral complexes. In a similar way the values of Yo for the tetrahedral complex CoC~PY2 were calculated for lower pyridine concentrations making use of solubility and spectral data. In turn, the same dependence of Y0 on solvent compos
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