SPECTROCHEMISTRY OF SOLUTIONS .22. A STUDY OF THE COMPLEXATION OF ZINC(II), CADMIUM(II) AND MERCURY(II) BY CYANIDE IN LIQUID-AMMONIA BY VIBRATIONAL SPECTROSCOPY

The stepwise ligation patterns for the complexation by CN- of Zn2+, Cd2+ and Hg2+ in liquid ammonia solutions have been established from analyses of the infrared and Raman spectra of M(II)/CN- mixtures between 295 and 213 K. All the cyano-complexes are soluble and complexation, complete at the 4:1 stage for all three metal cations, proceeds by direct replacement of the solvating NH3 by CN- on tetrahedrally co-ordinated Zn2+ and Hg2+. Cyano-ligation of Cd2+ is quite different with octahedral 1:1 and 2:1 complexes, followed by a change to tetrahedral 3:1 and 4:1 complexes. There is no evidence for multinuclear complex formation, and mononuclear complexation is strong for all three metals; no uncomplexed CN- is present until the solution composition exceeds [CN-]/[M(II)] = 4.0:1. Stable outer-sphere complexes (or ion associates) form between the 4:1 complexes and the Na+ cocation, viz. [Na+ ... -NC-M(CN)3], and there is some evidence for similar association of the 3:1 complex of Hg2+, i.e. [Na+ ... -NC-M(CN)2(NH3)]. Assignments of the V(sym)(CN), V(asym)(CN) and v(M-N) stretching frequencies of the complexes are presented.