The dimerization of the ion pairs MNCS (M = Li, Na, K) has been investigated in tetrahydrofuran (THF) and in 1,3-dixolane (DXL) solutions by infrared and Raman spectroscopy. The mixed dimer LiNa(NCS)2 and the triple cations Li2NCS+ and Na2NCS+ have also been characterized. The three dimers (MNCS)2 have the same rhombic structure. Dimerization reactions exhibit a nonideal behavior which mostly comes from ion pair-ion pair interactions. The dimerization is more pronounced for NaNCS and KNCS than for LiNCS, In all cases these reactions are athermic or endothermic, i.e., entropy controlled. The upward curvature which is observed when the logarithm of the solubility of MSCN is plotted against the reciprocal of the temperature is explained by the dimerization and by ion pair-ion pair interactions.
Solutions of LiSCN in donor solvent – benzene mixtures have been investigated by solubility measurements and by infrared spectrometry. The state of aggregation of LiSCN has been obtained from the ir spectra in the ν(CN) region 2100–2000 cm−1. The selected solvents (tetrahydrofuran, pyridine, acetone, diethylcarbonate, butylacetate, ethylmethylketone) are more or less associating towards LiSCN. The addition of benzene causes an increase of the associating ability of the solvents. By changing the donor solvent and the amount of benzene it has been possible to obtain the ion pair LiNCS, the dimer (LiNCS)2, and higher aggregates in various proportions. The solvation number of lithium has been measured in solvent vibration regions. Each step of aggregation is accompanied by lithium desolvation. The coordination of lithium in all the species which are formed remains equal to 4, or slightly lower in some cases.
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