Complexes between alkaline-earth-metal ions M*' and cyanate OCN' in aprotic solvents are studied by infrared spectrometry in the v(CN) region (2150-2250 cm-'). In dimethylsulphoxide (DMSO) and hexamethylphosphotriamide (HMPT) isocyanate MNCO ion pairs exhibit a specific v(CN) band while M(NCO)z species usually have two v(CN) bands. Pure alkaline earth cyanates are in part dissociated into MNCO' and OCN-in DMSO. When they are soluble they are not dissociated in N,N-dimethylformamide (DMF) and in pyridine.
The Mg"-0CK system in HMPT is the only case where two different ion pairs are observed: pl at 2205 cm-' and pz at 2213 cm". The high temperature species p2 is tighter than p,, The reaction pl -+ pz is a desolvation reaction as shown by the large values of AH and AS (AH = 37 kJ.mol-' and AS = 100 J.K-'.mol-I).
Similar results are obtained with other non-transition ions (A?', Zn*', Cd", Hg", Ag', Tl'). The Av(CN) wavenumber shift of an ion pair relative to free OCK (= 2 137 cm-') is always positive and roughly proportional to the polarizing power of M". This is in agreement with an isocyanate bonding MNCO. When M"' is a sotI cation Av(CN) is smaller than expected. This effect is common to all cases of M...CN bonding. The i.r. spectrum of AgOCN in tetrahydrothiophene (THT) exhibits two bands at 2162 cm-' (AgNCO) and 2250 cm-'. The second band is assigned to a bridging group AgOCNAg in (AgNCO)z.