The discovery of the crown ethers was followed by synthesis of macro bicyclic poly ethers containing three poly ether strands joined by two bridge head nitrogen atoms [1]. These compounds have three-dimentional cavites which can accommodate a metal ion of a suitable size and from an inclusion complex. These ligands which developed by Lehn and his– co-workers [2], were called 2 – cryptands where 2 indicates the bicyclic ligand such as kryptofix-222. It is important to mention that the marocyclic crown ethers have many applications such as in biological activity, corrosion chemistry, analytical chem-istry, phase – transfer catalysis and industrial production such as nuclear energy, electronics and electro-chemical photosensitive materials [3-4].For better understanding of the interactions between macrocyclic crown ethers and metal cations in solutions, it requires the study of various parameters govering these interactions. The thermodynamic studies of these interactions give important informations about their complexation reactions and the selectivities of these ligands towards different metal cations [5].In this paper,we report the results of a conductance study of the intractions between Co2+,Ni2+,Cu2+ metal cations with crytand 222 in accetonitrile-ethylacetate binary mixtures at various temperature .The conductance data show that in most cases ,the stoichiometry of the complexes ,is 1:1 [M:L], but in the case of complexation of Co2+and Cu2+ cations with C222 , in addition of 1:1 [M:L] complexes 1:2 [M:L2] complexes are formed in some solvent systems . The stability constants of the 1:1[M: L] complexes were determined using a computer program,GENPLOT. The stability order of complexes between C222 and Co2+, Ni2+and Cu2+ cations at 250C in the binary solution of AN-EtOAc(mol % AN 20) is :(C222. Co)2+> (C222. Ni) 2+> (C222. Cu) 2+,but in case of another AN-EtOAc binary solvent solution (mol % AN 25) it was found to be : (C222. Ni) 2+ > (C222. Co) 2+> (C222. Cu) 2+ and in case of AN-EtOAC binary mixed systems (mol % AN 50 and 75) the stability order changes as : (C222. Cu) 2+> (C222. Ni) 2+> (C222. Co) 2+.The order of the stability of the complexes in pure AN is : (C222. Cu) 2+ > (C222. Co) 2+> (C222. Ni) 2+. The results obtained in this study ,reveal that the selectivity of cryptand 222 for the studied cations in binary mixed non-aqueos solvents changes with the nature and composition of the solvent system.A non-linear behavior was observed for variation of the stability constants (log K) of complexes versus the composition of the binary solvent solution.This behavior is due to solvent-solvent interaction and also it may be due to preferential solvation of the cation and ligand in the binary mixed solvent system .The value of thermodynamic quantities,(ΔH0 C,ΔS0 C) show that in most solvent systems , the studied cryptates are enthropy stabilized but with the exception of a few cases , they are enthalpy destabilized .

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