A CALORIMETRIC STUDY OF THE PROTONATION OF VITAMIN-D(3) AND OF 3 OTHER LIGANDS AND THEIR FORMATION OF BINARY AND TERNARY COMPLEXES WITH CD(II)

The heats of protonation of vitamin D3 (VD3) and of glycine (gly), 2,2 '-bipyridyl (bipy) and ethylenediamine (en), and their coordination wi th Cd(II) to form binary and ternary complexes have been determined at 37-degrees-C and I = 0.1 M (KNO3) in 70% (v/v) ethanol, using a model RD-1 automatic conduction calorimeter. The entropies of formation of these systems were calculated. The results show that all the ternary s ystems have large positive entropy values. It seems that the stacking effect between the alkyl residue, which is far removed from the coordi nation atom (0) in the VD3 molecule, and the aromatic ring of bipy or the carbon linkage of gly (or en) plays a very important role in the e ntropy increase of the ternary complex compounds. Furthermore, the mag nitude of the stacking effect may be in the order aryl-aryl > aryl-alk yl ring > alkyl ring-alkyl ring > alkyl ring-carbon linkage.