A study of ternary metal-xanthosine complexes in solution-Thermodynamic aspects

Abstract

The stability constants of the ternary complexes (1:1:1) of Cu(II), Ni(II), Zn(II), Co(II), Mn(II), Mg(II) and Ca(II) with xanthosine and a secondary ligand (histidine, histamine, catechol and glycine) been determined potentiometrically at 25, 35 and 45°C at an ionic strength μ = 0.10 <em>M </em>(KNO₃). These ternary complexes have been found to be more stable than the corresponding binary complexes as shown by the positive value of <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0>log <em>K</em>. The enthalpy (<img src='http://www.niscair.res.in/jinfo/delta.gif' border=0> <em>H</em><img src='http://www.niscair.res.in/jinfo/equation128.gif' border=0>) changes associated with the ternary complexes of xanthosine have been calculated from the 1:1:1 stability data by temperature coefficient method. With the help of these thermodynamic parameters, we have evaluated <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><img src='http://www.niscair.res.in/jinfo/delta.gif' border=0> <em>H</em><img src='http://www.niscair.res.in/jinfo/equation128.gif' border=0> and <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><img src='http://www.niscair.res.in/jinfo/delta.gif' border=0> <em>S</em><img src='http://www.niscair.res.in/jinfo/equation128.gif' border=0>, to explain the extra stability found in these ternary complexes.