Steric solvent effect on small and large cations: Calorimetric study of halogeno and thiocyanato complexes of beryllium(II) and cadmium(II) in N,N-dimethylacetamide

Solution equilibria of beryllium(II) and cadmium(II) complexes with halide and thiocyanate ions are studied in N,N-dimethylacetamide (DMA) by calorimetry. Mononuclear Be^II complexes, [BeX]⁺ and [BeX₂] (X = Cl, SCN), and Cd^II complexes, [CdX]⁺, [CdX₂], [CdX₃]^- and [CdX₄]^2- (X = Cl, Br, I, SCN), are found, and their formation constants, enthalpies and entropies are determined. The Be^II systems are also studied in N,N-dimethylformamide (DMF). Each of the Be^II complexes has very similar thermodynamic quantities in DMA and in DMF, in sharp contrast to first-row transition metal(II) complexes where a marked solvent effect has been observed. In spite of the very small size of the cation, steric hindrance of DMA is absent in the four-coordinate solvation structure of Be²⁺. ΔH_n° and ΔS_n° for the Cd^II complexes in DMA are largely different from those in DMF, which shows the steric solvent effect of DMA, i.e. steric hindrance does exist in the six-coordinate structure of this fairly large cation. When combined, these results offer clear evidence for the octahedral-specific nature of the steric hindrance in metal-DMA interactions.