The determination of formation constants of binary inner transition metal complexes where M=Y(III) or La(III) or Ce(III) or Pr(III) or Nd(III) or Sm (III) or Gd (III) or Dy (III) or Th(IV) andL = Creatinine have been carried out using Irving–Rossotti titration technique in aqueous mediaat different temperatures and at ionic strength. To understand more about the nature ofequilibrium involving inner transition metals with Creatinine, the effect of dielectric constants onthe stability of these complexes at different percentage of solvent variation and at differentsolvent systems has been studied. The formation constant (logβn) have been calculated on IBMcomputer using BEST Program. Thermodynamic parameters (ΔG, ΔH and ΔS) are alsoevaluated, negative ΔG, ΔH and ΔS values indicate that complex formation is favorable atordinary temperatures. Species distribution curves of complexes have been plotted as function ofpH using Fortran IV program SPE PLOT to visualize the equilibria systems in pH range of 2-8pH. The order of stability for metals is Y < La < Ce < Pr < Nd < Sm < Gd < Dy < Th. This ordercan be explained on the basis of basicity of ligand, protonation of ligand, electronicconfiguration of metal ions, size and ionic potential of tripositive ion, charge/size ratio of metalions and species distribution diagrams. پرونده مقاله