A series of distannyl derivatives, 1-9, were studied for their application as phosphate-selective carriers in polymer-based liquid membranes. A drastically different potentiometric behavior was observed depending on the distance between the tin(1V) coordinating centers and the ligands attached to them. Some of the compounds examined, i.e. (PhSnBrz),CH, (1) and (Me3SiCHZSnCl2CH2),CH2 (3) showed very high potentiometric selectivity towards phosphate over other lipophilic anions such as perchlorate and thiocyanate. Results indicate that I) the optimal number of CH, groups between the Sn-centers are either one or three, 2) the electron-withdrawing power of the organic ligands attached to Sn-centers strongly influences the overall response towards phosphate, and 3) the steric effect of the organic substituents is important in the potentiometric selectivity observed. The design of a carrier for selective recognition of orthophosphates is not a trivial matter, due mainly to two reasons: a ) The free energy of hydration of orthophosphate (ca. -95" cal/mol) [6] is much smaller than that of other oxoanions such as perchlorate (cu. -50" cal/mol) [7] and b ) the large size of this anion prohibits the use of size-exclusion principles for increased selectivity. For these reasons, and to design a phosphate carrier, the sum of the free energy of interaction or ligation with the carrier combined with the negative free energy of hydration has to be higher than that of any interfering ions. Another difficulty arises from the fact that the tetrahedral geometry of the orthophosphate ion closely resembles that of the perchlorate and sulfate while the sizes of all three are very similar. The selective phosphate carrier would thus have to have certain distinct characteristics which are: a ) a strong ligation energy towards orthophosphate to overcome its high negative energy of hydration, and b ) the right shape of accommodate the ion and to take advantage of the multidentate effect.There are two characteristics of the phosphate anion that can be utilized for the design of a phosphate carrier, which are: a ) the formation of multiple H-bonding between the four 0-atoms of the phosphate and any surrounding H-atoms [8], and b ) the ability of the phosphate 0-atoms to form stronger metal complexes than those with similar structures such as perchlorate and sulfatewith the order being phosphate > perchlorate > sulfate [9].

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