Aggregation of alkaline-earth-metal thiocyanates in aprotic donor solvents

Abstract

Alkaline-earth-metal thiocyanates M(SCN)₂(M = Mg, Ca, Sr and Ba) in non-aqueous solution have been investigated by IR and Raman spectroscopy. The following aprotic solvents given in the order of increasing associating power have been used: N, N-dimethylformamide (DMF), acetone, acetonitrile (MeCN), tetrahydrofuran (THF) and methylacetate (MeCO₂Me). These salts are in part dissociated in DMF while in the other solvents the predominant species are neutral molecules M(NCS)₂ and dimers. The three dimers which have been identified are Ca₂(NCS)₄, Sr₂(NCS)₄ and Ba₂(NCS)₄. The stability of these species is ensured by N–_µ2 SCN bridges between both cations, and increases in the order Ca < Sr < Ba. The thermodynamic investigation of the ion pairing of Mg²⁺ with SCN^– in DMF and of the dimerization of Ba(NCS)₂ in THF shows that both reactions are entropy-driven processes.