Vibrational study of ionic association in aprotic solvents. Part 12. Isothiocyanate M(NCS) complexes of non-transition-metal ions

Abstract

Isothiocyanate complexes M (NCS)(ion pairs) formed between SCN^– and non-transition-metal cations M^z+(Li⁺, Na⁺, Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺, Cd²⁺, Al³⁺, Ga³⁺, ln³⁺, Tl⁺, Pb²⁺, or Bi³⁺) have been investigated in polar aprotic solvents (dimethyl sulphoxide or dimethylformamide). The effects of ion pairing on the CN and CS vibrations of SCN were studied by i.r. and Raman spectroscopy. The ν(CS) frequency is well correlated to the polarizing power P of M^Z+ while ν(CN) also depends on its polarizability α. As these two factors act on ν(CN) in opposite directions, the frequency shift in M (NCS) relative to SCN^– is negative when α is large. The ν(CS) frequency shift is interpreted by the mesomeric effect while other factors also contribute to the ν(CN) frequency shift: stabilization of σ orbitals of SCN^– by M^z+, transfer of π electrons to M^z+, and back donation of electrons from M^z+ to π* antibonding orbitals of the CN group. The i.r. intensity of the CN vibration is also correlated to P and α and in that case both factors act in the same direction. An electrostatic model of the ion pair gives results in agreement with the observed intensity enhancements. It is shown that these interpretations are also useful in the case of transition-metal complexes.