Spectrochemistry of solutions. Part 22. A study of the complexation of zinc(II), cadmium(II) and mercury(II) by cyanide in liquid ammonia by vibrational spectroscopy

Abstract

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.