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