Substitution on Complexes of Alkali and Alkaline Earth Metal Ions

Ligand Substitution on Complexes of Alkali and Alkaline Earth Metal Ions 

The exchange of alkali metal ions in crown ether complexes can proceed through either a unimolecular or a bimolecular mechanism as shown in Eqs. (3) and (4), respectively, and the factors affecting the operation of these mechanisms have been investigated in a Na NMR study. When the crown ether (C) is 18C6 

The role of counterions in the bimolecular exchange of Na in [Na DB24C8] in nitromethane solvent has also been investigated by Na NMR methods. As Na has been shown to be completely isolated from direct solvent and counterion interaction when it is complexed by DB24C8 (2,3,5,6,8,9,14,15,17,18,20,21-dodecahydro-1,4,7,10,13,16,19,22-octaoxadibenzocy clotetracosadiene) it is 

The ultrasonic relaxation kinetic technique measures faster reactions than the NMR method and thus is able to detect more steps in the crown ether complexation sequence. This is exemplified by a study of the complexation of LiC104 by 18C6 in propylene carbonate in which two relaxations are detected which are attributed to the operation of an Eigen-Winkler mechanism for Li exchange as in Eq. (5). 

Li ---18C6 represents an outer-sphere complex and [Li 18C6 ] represents a complex in which the solvation and the crown ether conformation are in an intermediate state on the way to the final ground state, [Li-18C6]. The exchange process is characterized by 2 = L45 x 10 s and = 6.3 x 10 s at 298.2 

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